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NAME

       perf_event_open - set up performance monitoring

SYNOPSIS

       #include <linux/perf_event.h>
       #include <linux/hw_breakpoint.h>

       int perf_event_open(struct perf_event_attr *attr,
                           pid_t pid, int cpu, int group_fd,
                           unsigned long flags);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

       Given  a  list  of  parameters,  perf_event_open()  returns  a file descriptor, for use in
       subsequent system calls (read(2), mmap(2), prctl(2), fcntl(2), etc.).

       A call to perf_event_open() creates a file descriptor that  allows  measuring  performance
       information.  Each file descriptor corresponds to one event that is measured; these can be
       grouped together to measure multiple events simultaneously.

       Events can be enabled and disabled in two ways: via ioctl(2) and via  prctl(2).   When  an
       event  is  disabled it does not count or generate overflows but does continue to exist and
       maintain its count value.

       Events come in two flavors: counting and sampled.  A counting event is one  that  is  used
       for  counting  the  aggregate  number  of  events  that occur.  In general, counting event
       results  are  gathered  with  a  read(2)  call.   A  sampling  event  periodically  writes
       measurements to a buffer that can then be accessed via mmap(2).

   Arguments
       The pid and cpu arguments allow specifying which process and CPU to monitor:

       pid == 0 and cpu == -1
              This measures the calling process/thread on any CPU.

       pid == 0 and cpu >= 0
              This measures the calling process/thread only when running on the specified CPU.

       pid > 0 and cpu == -1
              This measures the specified process/thread on any CPU.

       pid > 0 and cpu >= 0
              This measures the specified process/thread only when running on the specified CPU.

       pid == -1 and cpu >= 0
              This   measures   all  processes/threads  on  the  specified  CPU.   This  requires
              CAP_SYS_ADMIN capability or a /proc/sys/kernel/perf_event_paranoid  value  of  less
              than 1.

       pid == -1 and cpu == -1
              This setting is invalid and will return an error.

       When  pid  is  greater  than zero, permission to perform this system call is governed by a
       ptrace access mode PTRACE_MODE_READ_REALCREDS check; see ptrace(2).

       The group_fd argument allows event groups to be created.  An event  group  has  one  event
       which  is the group leader.  The leader is created first, with group_fd = -1.  The rest of
       the group members are created with subsequent perf_event_open() calls with group_fd  being
       set  to  the  file  descriptor of the group leader.  (A single event on its own is created
       with group_fd = -1 and is considered to be a group with only 1 member.)  An event group is
       scheduled onto the CPU as a unit: it will be put onto the CPU only if all of the events in
       the group can be put onto the CPU.  This means that the values of the member events can be
       meaningfully  compared—added,  divided  (to  get ratios), and so on—with each other, since
       they have counted events for the same set of executed instructions.

       The flags argument is formed by ORing together zero or more of the following values:

       PERF_FLAG_FD_CLOEXEC (since Linux 3.14)
              This flag enables the close-on-exec flag for the created event file descriptor,  so
              that  the file descriptor is automatically closed on execve(2).  Setting the close-
              on-exec flags at creation time, rather than later with fcntl(2),  avoids  potential
              race  conditions where the calling thread invokes perf_event_open() and fcntl(2) at
              the same time as another thread calls fork(2) then execve(2).

       PERF_FLAG_FD_NO_GROUP
              This flag tells the event to ignore the group_fd parameter except for  the  purpose
              of setting up output redirection using the PERF_FLAG_FD_OUTPUT flag.

       PERF_FLAG_FD_OUTPUT (broken since Linux 2.6.35)
              This  flag  re-routes the event's sampled output to instead be included in the mmap
              buffer of the event specified by group_fd.

       PERF_FLAG_PID_CGROUP (since Linux 2.6.39)
              This flag activates  per-container  system-wide  monitoring.   A  container  is  an
              abstraction  that  isolates  a  set  of  resources for finer-grained control (CPUs,
              memory, etc.).  In this mode, the event is measured only if the thread  running  on
              the  monitored  CPU  belongs  to  the designated container (cgroup).  The cgroup is
              identified by passing a file descriptor opened on its  directory  in  the  cgroupfs
              filesystem.   For  instance,  if  the cgroup to monitor is called test, then a file
              descriptor opened on /dev/cgroup/test (assuming cgroupfs is mounted on /dev/cgroup)
              must  be  passed  as  the  pid  parameter.  cgroup monitoring is available only for
              system-wide events and may therefore require extra permissions.

       The perf_event_attr structure provides detailed configuration information  for  the  event
       being created.

           struct perf_event_attr {
               __u32 type;                 /* Type of event */
               __u32 size;                 /* Size of attribute structure */
               __u64 config;               /* Type-specific configuration */

               union {
                   __u64 sample_period;    /* Period of sampling */
                   __u64 sample_freq;      /* Frequency of sampling */
               };

               __u64 sample_type;  /* Specifies values included in sample */
               __u64 read_format;  /* Specifies values returned in read */

               __u64 disabled       : 1,   /* off by default */
                     inherit        : 1,   /* children inherit it */
                     pinned         : 1,   /* must always be on PMU */
                     exclusive      : 1,   /* only group on PMU */
                     exclude_user   : 1,   /* don't count user */
                     exclude_kernel : 1,   /* don't count kernel */
                     exclude_hv     : 1,   /* don't count hypervisor */
                     exclude_idle   : 1,   /* don't count when idle */
                     mmap           : 1,   /* include mmap data */
                     comm           : 1,   /* include comm data */
                     freq           : 1,   /* use freq, not period */
                     inherit_stat   : 1,   /* per task counts */
                     enable_on_exec : 1,   /* next exec enables */
                     task           : 1,   /* trace fork/exit */
                     watermark      : 1,   /* wakeup_watermark */
                     precise_ip     : 2,   /* skid constraint */
                     mmap_data      : 1,   /* non-exec mmap data */
                     sample_id_all  : 1,   /* sample_type all events */
                     exclude_host   : 1,   /* don't count in host */
                     exclude_guest  : 1,   /* don't count in guest */
                     exclude_callchain_kernel : 1,
                                           /* exclude kernel callchains */
                     exclude_callchain_user   : 1,
                                           /* exclude user callchains */
                     mmap2          :  1,  /* include mmap with inode data */
                     comm_exec      :  1,  /* flag comm events that are
                                              due to exec */
                     use_clockid    :  1,  /* use clockid for time fields */
                     context_switch :  1,  /* context switch data */

                     __reserved_1   : 37;

               union {
                   __u32 wakeup_events;    /* wakeup every n events */
                   __u32 wakeup_watermark; /* bytes before wakeup */
               };

               __u32     bp_type;          /* breakpoint type */

               union {
                   __u64 bp_addr;          /* breakpoint address */
                   __u64 config1;          /* extension of config */
               };

               union {
                   __u64 bp_len;           /* breakpoint length */
                   __u64 config2;          /* extension of config1 */
               };
               __u64 branch_sample_type;   /* enum perf_branch_sample_type */
               __u64 sample_regs_user;     /* user regs to dump on samples */
               __u32 sample_stack_user;    /* size of stack to dump on
                                              samples */
               __s32 clockid;              /* clock to use for time fields */
               __u64 sample_regs_intr;     /* regs to dump on samples */
               __u32 aux_watermark;        /* aux bytes before wakeup */
               __u16 sample_max_stack;     /* max frames in callchain */
               __u16 __reserved_2;         /* align to u64 */

           };

       The fields of the perf_event_attr structure are described in more detail below:

       type   This field specifies the overall event type.  It has one of the following values:

              PERF_TYPE_HARDWARE
                     This  indicates  one  of  the  "generalized" hardware events provided by the
                     kernel.  See the config field definition for more details.

              PERF_TYPE_SOFTWARE
                     This indicates one of the software-defined events  provided  by  the  kernel
                     (even if no hardware support is available).

              PERF_TYPE_TRACEPOINT
                     This   indicates   a   tracepoint   provided   by   the   kernel  tracepoint
                     infrastructure.

              PERF_TYPE_HW_CACHE
                     This indicates a  hardware  cache  event.   This  has  a  special  encoding,
                     described in the config field definition.

              PERF_TYPE_RAW
                     This indicates a "raw" implementation-specific event in the config field.

              PERF_TYPE_BREAKPOINT (since Linux 2.6.33)
                     This  indicates  a  hardware breakpoint as provided by the CPU.  Breakpoints
                     can be read/write accesses  to  an  address  as  well  as  execution  of  an
                     instruction address.

              dynamic PMU
                     Since  Linux 2.6.38, perf_event_open() can support multiple PMUs.  To enable
                     this, a value exported by the kernel can  be  used  in  the  type  field  to
                     indicate  which  PMU  to  use.   The  value to use can be found in the sysfs
                     filesystem:   there   is   a   subdirectory   per   PMU    instance    under
                     /sys/bus/event_source/devices.   In  each  subdirectory there is a type file
                     whose content is an integer that  can  be  used  in  the  type  field.   For
                     instance,  /sys/bus/event_source/devices/cpu/type contains the value for the
                     core CPU PMU, which is usually 4.

       size   The size of the perf_event_attr structure for forward/backward compatibility.   Set
              this  using  sizeof(struct  perf_event_attr)  to allow the kernel to see the struct
              size at the time of compilation.

              The related define PERF_ATTR_SIZE_VER0 is set to 64; this was the size of the first
              published  struct.   PERF_ATTR_SIZE_VER1  is  72,  corresponding to the addition of
              breakpoints in Linux  2.6.33.   PERF_ATTR_SIZE_VER2  is  80  corresponding  to  the
              addition  of branch sampling in Linux 3.4.  PERF_ATTR_SIZE_VER3 is 96 corresponding
              to  the  addition  of  sample_regs_user  and  sample_stack_user   in   Linux   3.7.
              PERF_ATTR_SIZE_VER4  is  104  corresponding  to the addition of sample_regs_intr in
              Linux  3.19.   PERF_ATTR_SIZE_VER5  is  112  corresponding  to  the   addition   of
              aux_watermark in Linux 4.1.

       config This  specifies  which  event  you  want,  in conjunction with the type field.  The
              config1 and config2 fields are also taken into account in cases where  64  bits  is
              not  enough  to  fully  specify  the event.  The encoding of these fields are event
              dependent.

              There are various ways to set the config field that are dependent on the  value  of
              the  previously  described  type field.  What follows are various possible settings
              for config separated out by type.

              If type is PERF_TYPE_HARDWARE, we are measuring one of the generalized hardware CPU
              events.  Not all of these are available on all platforms.  Set config to one of the
              following:

                   PERF_COUNT_HW_CPU_CYCLES
                          Total cycles.  Be wary of what happens during CPU frequency scaling.

                   PERF_COUNT_HW_INSTRUCTIONS
                          Retired instructions.  Be careful, these can  be  affected  by  various
                          issues, most notably hardware interrupt counts.

                   PERF_COUNT_HW_CACHE_REFERENCES
                          Cache  accesses.   Usually this indicates Last Level Cache accesses but
                          this may vary depending on your CPU.  This may include  prefetches  and
                          coherency messages; again this depends on the design of your CPU.

                   PERF_COUNT_HW_CACHE_MISSES
                          Cache  misses.  Usually this indicates Last Level Cache misses; this is
                          intended     to     be     used     in     conjunction     with     the
                          PERF_COUNT_HW_CACHE_REFERENCES event to calculate cache miss rates.

                   PERF_COUNT_HW_BRANCH_INSTRUCTIONS
                          Retired  branch  instructions.   Prior  to  Linux 2.6.35, this used the
                          wrong event on AMD processors.

                   PERF_COUNT_HW_BRANCH_MISSES
                          Mispredicted branch instructions.

                   PERF_COUNT_HW_BUS_CYCLES
                          Bus cycles, which can be different from total cycles.

                   PERF_COUNT_HW_STALLED_CYCLES_FRONTEND (since Linux 3.0)
                          Stalled cycles during issue.

                   PERF_COUNT_HW_STALLED_CYCLES_BACKEND (since Linux 3.0)
                          Stalled cycles during retirement.

                   PERF_COUNT_HW_REF_CPU_CYCLES (since Linux 3.3)
                          Total cycles; not affected by CPU frequency scaling.

              If type is PERF_TYPE_SOFTWARE, we are measuring software  events  provided  by  the
              kernel.  Set config to one of the following:

                   PERF_COUNT_SW_CPU_CLOCK
                          This reports the CPU clock, a high-resolution per-CPU timer.

                   PERF_COUNT_SW_TASK_CLOCK
                          This reports a clock count specific to the task that is running.

                   PERF_COUNT_SW_PAGE_FAULTS
                          This reports the number of page faults.

                   PERF_COUNT_SW_CONTEXT_SWITCHES
                          This  counts  context  switches.   Until  Linux  2.6.34, these were all
                          reported  as  user-space  events,  after  that  they  are  reported  as
                          happening in the kernel.

                   PERF_COUNT_SW_CPU_MIGRATIONS
                          This reports the number of times the process has migrated to a new CPU.

                   PERF_COUNT_SW_PAGE_FAULTS_MIN
                          This  counts  the  number  of minor page faults.  These did not require
                          disk I/O to handle.

                   PERF_COUNT_SW_PAGE_FAULTS_MAJ
                          This counts the number of major page faults.  These required  disk  I/O
                          to handle.

                   PERF_COUNT_SW_ALIGNMENT_FAULTS (since Linux 2.6.33)
                          This  counts  the  number  of  alignment  faults.   These  happen  when
                          unaligned memory accesses happen; the kernel can handle  these  but  it
                          reduces performance.  This happens only on some architectures (never on
                          x86).

                   PERF_COUNT_SW_EMULATION_FAULTS (since Linux 2.6.33)
                          This counts the number of emulation faults.  The kernel sometimes traps
                          on  unimplemented  instructions and emulates them for user space.  This
                          can negatively impact performance.

                   PERF_COUNT_SW_DUMMY (since Linux 3.12)
                          This is a placeholder event that counts nothing.  Informational  sample
                          record  types  such  as  mmap or comm must be associated with an active
                          event.   This  dummy  event  allows  gathering  such  records   without
                          requiring a counting event.

              If  type  is  PERF_TYPE_TRACEPOINT,  then we are measuring kernel tracepoints.  The
              value to use in config can be obtained from under debugfs tracing/events/*/*/id  if
              ftrace is enabled in the kernel.

              If  type  is  PERF_TYPE_HW_CACHE, then we are measuring a hardware CPU cache event.
              To calculate the appropriate config value use the following equation:

                      (perf_hw_cache_id) | (perf_hw_cache_op_id << 8) |
                      (perf_hw_cache_op_result_id << 16)

                  where perf_hw_cache_id is one of:

                      PERF_COUNT_HW_CACHE_L1D
                             for measuring Level 1 Data Cache

                      PERF_COUNT_HW_CACHE_L1I
                             for measuring Level 1 Instruction Cache

                      PERF_COUNT_HW_CACHE_LL
                             for measuring Last-Level Cache

                      PERF_COUNT_HW_CACHE_DTLB
                             for measuring the Data TLB

                      PERF_COUNT_HW_CACHE_ITLB
                             for measuring the Instruction TLB

                      PERF_COUNT_HW_CACHE_BPU
                             for measuring the branch prediction unit

                      PERF_COUNT_HW_CACHE_NODE (since Linux 3.1)
                             for measuring local memory accesses

                  and perf_hw_cache_op_id is one of:

                      PERF_COUNT_HW_CACHE_OP_READ
                             for read accesses

                      PERF_COUNT_HW_CACHE_OP_WRITE
                             for write accesses

                      PERF_COUNT_HW_CACHE_OP_PREFETCH
                             for prefetch accesses

                  and perf_hw_cache_op_result_id is one of:

                      PERF_COUNT_HW_CACHE_RESULT_ACCESS
                             to measure accesses

                      PERF_COUNT_HW_CACHE_RESULT_MISS
                             to measure misses

              If type is PERF_TYPE_RAW, then a custom "raw" config value is  needed.   Most  CPUs
              support  events  that  are  not  covered  by  the  "generalized" events.  These are
              implementation defined; see your CPU  manual  (for  example  the  Intel  Volume  3B
              documentation or the AMD BIOS and Kernel Developer Guide).  The libpfm4 library can
              be used to translate from the name in the architectural  manuals  to  the  raw  hex
              value perf_event_open() expects in this field.

              If type is PERF_TYPE_BREAKPOINT, then leave config set to zero.  Its parameters are
              set in other places.

       sample_period, sample_freq
              A "sampling" event is one that generates an overflow notification every  N  events,
              where  N  is given by sample_period.  A sampling event has sample_period > 0.  When
              an overflow occurs, requested data is recorded in the mmap buffer.  The sample_type
              field controls what data is recorded on each overflow.

              sample_freq  can  be used if you wish to use frequency rather than period.  In this
              case, you set the freq flag.  The kernel will adjust the sampling period to try and
              achieve the desired rate.  The rate of adjustment is a timer tick.

       sample_type
              The various bits in this field specify which values to include in the sample.  They
              will be recorded in a ring-buffer, which is available to user space using  mmap(2).
              The  order  in  which the values are saved in the sample are documented in the MMAP
              Layout subsection below; it is not the enum perf_event_sample_format order.

              PERF_SAMPLE_IP
                     Records instruction pointer.

              PERF_SAMPLE_TID
                     Records the process and thread IDs.

              PERF_SAMPLE_TIME
                     Records a timestamp.

              PERF_SAMPLE_ADDR
                     Records an address, if applicable.

              PERF_SAMPLE_READ
                     Record counter values for all events in a group, not just the group leader.

              PERF_SAMPLE_CALLCHAIN
                     Records the callchain (stack backtrace).

              PERF_SAMPLE_ID
                     Records a unique ID for the opened event's group leader.

              PERF_SAMPLE_CPU
                     Records CPU number.

              PERF_SAMPLE_PERIOD
                     Records the current sampling period.

              PERF_SAMPLE_STREAM_ID
                     Records a unique ID for the opened event.  Unlike PERF_SAMPLE_ID the  actual
                     ID  is  returned,  not  the  group  leader.   This ID is the same as the one
                     returned by PERF_FORMAT_ID.

              PERF_SAMPLE_RAW
                     Records additional data, if  applicable.   Usually  returned  by  tracepoint
                     events.

              PERF_SAMPLE_BRANCH_STACK (since Linux 3.4)
                     This  provides  a  record  of  recent  branches,  as  provided by CPU branch
                     sampling hardware (such as Intel Last  Branch  Record).   Not  all  hardware
                     supports this feature.

                     See  the  branch_sample_type  field  for  how  to  filter which branches are
                     reported.

              PERF_SAMPLE_REGS_USER (since Linux 3.7)
                     Records the current user-level CPU register state (the values in the process
                     before the kernel was called).

              PERF_SAMPLE_STACK_USER (since Linux 3.7)
                     Records the user level stack, allowing stack unwinding.

              PERF_SAMPLE_WEIGHT (since Linux 3.10)
                     Records  a  hardware  provided  weight  value  that expresses how costly the
                     sampled event was.  This allows the hardware to highlight  expensive  events
                     in a profile.

              PERF_SAMPLE_DATA_SRC (since Linux 3.10)
                     Records  the  data source: where in the memory hierarchy the data associated
                     with the sampled instruction came from.   This  is  available  only  if  the
                     underlying hardware supports this feature.

              PERF_SAMPLE_IDENTIFIER (since Linux 3.12)
                     Places  the SAMPLE_ID value in a fixed position in the record, either at the
                     beginning (for sample events) or at the end (if a non-sample event).

                     This was necessary because a sample stream may  have  records  from  various
                     different  event  sources  with different sample_type settings.  Parsing the
                     event stream properly was not possible because the format of the record  was
                     needed  to find SAMPLE_ID, but the format could not be found without knowing
                     what event the sample belonged to (causing a circular dependency).

                     The PERF_SAMPLE_IDENTIFIER setting makes the event stream always parsable by
                     putting SAMPLE_ID in a fixed location, even though it means having duplicate
                     SAMPLE_ID values in records.

              PERF_SAMPLE_TRANSACTION (since Linux 3.13)
                     Records reasons for transactional memory abort  events  (for  example,  from
                     Intel TSX transactional memory support).

                     The  precise_ip  setting  must  be greater than 0 and a transactional memory
                     abort event must be measured or no values will be recorded.  Also note  that
                     some  perf_event  measurements,  such  as  sampled cycle counting, may cause
                     extraneous aborts (by causing an interrupt during a transaction).

              PERF_SAMPLE_REGS_INTR (since Linux 3.19)
                     Records a  subset  of  the  current  CPU  register  state  as  specified  by
                     sample_regs_intr.   Unlike  PERF_SAMPLE_REGS_USER  the  register values will
                     return kernel register state if the overflow happened while kernel  code  is
                     running.   If  the  CPU  supports hardware sampling of register state (i.e.,
                     PEBS on Intel x86) and precise_ip is set higher than zero then the  register
                     values  returned  are  those captured by hardware at the time of the sampled
                     instruction's retirement.

       read_format
              This  field  specifies  the  format  of  the  data  returned  by   read(2)   on   a
              perf_event_open() file descriptor.

              PERF_FORMAT_TOTAL_TIME_ENABLED
                     Adds the 64-bit time_enabled field.  This can be used to calculate estimated
                     totals if the PMU is overcommitted and multiplexing is happening.

              PERF_FORMAT_TOTAL_TIME_RUNNING
                     Adds the 64-bit time_running field.  This can be used to calculate estimated
                     totals if the PMU is overcommitted and multiplexing is happening.

              PERF_FORMAT_ID
                     Adds a 64-bit unique value that corresponds to the event group.

              PERF_FORMAT_GROUP
                     Allows all counter values in an event group to be read with one read.

       disabled
              The  disabled bit specifies whether the counter starts out disabled or enabled.  If
              disabled, the event can later be enabled by ioctl(2), prctl(2), or enable_on_exec.

              When creating an event group,  typically  the  group  leader  is  initialized  with
              disabled  set  to  1  and  any child events are initialized with disabled set to 0.
              Despite disabled being 0, the child events will not start until the group leader is
              enabled.

       inherit
              The  inherit  bit specifies that this counter should count events of child tasks as
              well as the task specified.  This applies only to new children, not to any existing
              children  at  the  time the counter is created (nor to any new children of existing
              children).

              Inherit does not  work  for  some  combinations  of  read_format  values,  such  as
              PERF_FORMAT_GROUP.

       pinned The  pinned  bit  specifies  that the counter should always be on the CPU if at all
              possible.  It applies only to hardware counters and only to group  leaders.   If  a
              pinned  counter  cannot  be  put  onto  the CPU (e.g., because there are not enough
              hardware counters or because of a conflict with some other event), then the counter
              goes  into  an 'error' state, where reads return end-of-file (i.e., read(2) returns
              0) until the counter is subsequently enabled or disabled.

       exclusive
              The exclusive bit specifies that when this counter's group is on the CPU, it should
              be  the  only  group  using  the  CPU's  counters.   In  the  future this may allow
              monitoring programs to support PMU features that need to run alone so that they  do
              not disrupt other hardware counters.

              Note  that many unexpected situations may prevent events with the exclusive bit set
              from ever running.  This includes any users running a  system-wide  measurement  as
              well  as any kernel use of the performance counters (including the commonly enabled
              NMI Watchdog Timer interface).

       exclude_user
              If this bit is set, the count excludes events that happen in user space.

       exclude_kernel
              If this bit is set, the count excludes events that happen in kernel space.

       exclude_hv
              If this bit is set, the count excludes events that happen in the hypervisor.   This
              is  mainly  for  PMUs that have built-in support for handling this (such as POWER).
              Extra support is needed for handling hypervisor measurements on most machines.

       exclude_idle
              If set, don't count when the CPU is idle.

       mmap   The mmap bit enables generation of PERF_RECORD_MMAP samples for every mmap(2)  call
              that  has  PROT_EXEC  set.   This  allows tools to notice new executable code being
              mapped into a program (dynamic shared libraries for example) so that addresses  can
              be mapped back to the original code.

       comm   The  comm  bit  enables tracking of process command name as modified by the exec(2)
              and prctl(PR_SET_NAME) system calls as well as writing to /proc/self/comm.  If  the
              comm_exec  flag is also successfully set (possible since Linux 3.16), then the misc
              flag PERF_RECORD_MISC_COMM_EXEC can be used to differentiate the exec(2) case  from
              the others.

       freq   If this bit is set, then sample_frequency not sample_period is used when setting up
              the sampling interval.

       inherit_stat
              This bit enables saving of event counts on  context  switch  for  inherited  tasks.
              This is meaningful only if the inherit field is set.

       enable_on_exec
              If this bit is set, a counter is automatically enabled after a call to exec(2).

       task   If this bit is set, then fork/exit notifications are included in the ring buffer.

       watermark
              If  set,  have  an  overflow notification happen when we cross the wakeup_watermark
              boundary.  Otherwise, overflow notifications happen after wakeup_events samples.

       precise_ip (since Linux 2.6.35)
              This controls the amount of skid.  Skid is how many instructions execute between an
              event of interest happening and the kernel being able to stop and record the event.
              Smaller skid  is  better  and  allows  more  accurate  reporting  of  which  events
              correspond to which instructions, but hardware is often limited with how small this
              can be.

              The possible values of this field are the following:

              0  SAMPLE_IP can have arbitrary skid.

              1  SAMPLE_IP must have constant skid.

              2  SAMPLE_IP requested to have 0 skid.

              3  SAMPLE_IP   must   have   0    skid.     See    also    the    description    of
                 PERF_RECORD_MISC_EXACT_IP.

       mmap_data (since Linux 2.6.36)
              This   is   the  counterpart  of  the  mmap  field.   This  enables  generation  of
              PERF_RECORD_MMAP samples for mmap(2) calls that do  not  have  PROT_EXEC  set  (for
              example data and SysV shared memory).

       sample_id_all (since Linux 2.6.38)
              If set, then TID, TIME, ID, STREAM_ID, and CPU can additionally be included in non-
              PERF_RECORD_SAMPLEs if the corresponding sample_type is selected.

              If PERF_SAMPLE_IDENTIFIER is specified, then an additional ID value is included  as
              the  last  value  to ease parsing the record stream.  This may lead to the id value
              appearing twice.

              The layout is described by this pseudo-structure:

                  struct sample_id {
                      { u32 pid, tid; }   /* if PERF_SAMPLE_TID set */
                      { u64 time;     }   /* if PERF_SAMPLE_TIME set */
                      { u64 id;       }   /* if PERF_SAMPLE_ID set */
                      { u64 stream_id;}   /* if PERF_SAMPLE_STREAM_ID set  */
                      { u32 cpu, res; }   /* if PERF_SAMPLE_CPU set */
                      { u64 id;       }   /* if PERF_SAMPLE_IDENTIFIER set */
                  };
                  ,in

       exclude_host (since Linux 3.2)
              When conducting measurements that include processes  running  VM  instances  (i.e.,
              have  executed  a  KVM_RUN  ioctl(2)), only measure events happening inside a guest
              instance.  This is only meaningful outside the guests; this setting does not change
              counts gathered inside of a guest.  Currently, this functionality is x86 only.

       exclude_guest (since Linux 3.2)
              When  conducting  measurements  that  include processes running VM instances (i.e.,
              have executed a KVM_RUN ioctl(2)), do not measure  events  happening  inside  guest
              instances.   This  is  only  meaningful  outside  the guests; this setting does not
              change counts gathered inside of a guest.  Currently,  this  functionality  is  x86
              only.

       exclude_callchain_kernel (since Linux 3.7)
              Do not include kernel callchains.

       exclude_callchain_user (since Linux 3.7)
              Do not include user callchains.

       mmap2 (since Linux 3.16)
              Generate  an  extended  executable  mmap  record  that  contains  enough additional
              information to uniquely identify shared mappings.  The mmap flag must also  be  set
              for this to work.

       comm_exec (since Linux 3.16)
              This  is  purely  a feature-detection flag, it does not change kernel behavior.  If
              this  flag  can  successfully  be  set,   then,   when   comm   is   enabled,   the
              PERF_RECORD_MISC_COMM_EXEC  flag  will  be  set  in the misc field of a comm record
              header if the rename event being reported was caused by a call  to  exec(2).   This
              allows tools to distinguish between the various types of process renaming.

       use_clockid (since Linux 4.1)
              This  allows selecting which internal Linux clock to use when generating timestamps
              via the clockid field.  This can make it easier to correlate perf sample times with
              timestamps generated by other tools.

       context_switch (since Linux 4.3)
              This  enables  the  generation  of PERF_RECORD_SWITCH records when a context switch
              occurs.  It also enables the generation of PERF_RECORD_SWITCH_CPU_WIDE records when
              sampling  in  CPU-wide  mode.   This  functionality  is  in  addition  to  existing
              tracepoint and software events for measuring context switches.   The  advantage  of
              this   method   is   that   it   will   give  full  information  even  with  strict
              perf_event_paranoid settings.

       wakeup_events, wakeup_watermark
              This union sets how many samples (wakeup_events) or bytes (wakeup_watermark) happen
              before  an  overflow  notification  happens.   Which one is used is selected by the
              watermark bit flag.

              wakeup_events counts only PERF_RECORD_SAMPLE record  types.   To  receive  overflow
              notification for all PERF_RECORD types choose watermark and set wakeup_watermark to
              1.

              Prior  to  Linux  3.0,  setting  wakeup_events  to  0  resulted  in   no   overflow
              notifications; more recent kernels treat 0 the same as 1.

       bp_type (since Linux 2.6.33)
              This chooses the breakpoint type.  It is one of:

              HW_BREAKPOINT_EMPTY
                     No breakpoint.

              HW_BREAKPOINT_R
                     Count when we read the memory location.

              HW_BREAKPOINT_W
                     Count when we write the memory location.

              HW_BREAKPOINT_RW
                     Count when we read or write the memory location.

              HW_BREAKPOINT_X
                     Count when we execute code at the memory location.

              The values can be combined via a bitwise or, but the combination of HW_BREAKPOINT_R
              or HW_BREAKPOINT_W with HW_BREAKPOINT_X is not allowed.

       bp_addr (since Linux 2.6.33)
              This is the address of the breakpoint.  For  execution  breakpoints,  this  is  the
              memory  address  of the instruction of interest; for read and write breakpoints, it
              is the memory address of the memory location of interest.

       config1 (since Linux 2.6.39)
              config1 is used for setting events that need an extra register or otherwise do  not
              fit     in     the     regular     config    field.     Raw    OFFCORE_EVENTS    on
              Nehalem/Westmere/SandyBridge use this field on Linux 3.3 and later kernels.

       bp_len (since Linux 2.6.33)
              bp_len  is  the  length   of   the   breakpoint   being   measured   if   type   is
              PERF_TYPE_BREAKPOINT.    Options   are   HW_BREAKPOINT_LEN_1,  HW_BREAKPOINT_LEN_2,
              HW_BREAKPOINT_LEN_4, and HW_BREAKPOINT_LEN_8.  For  an  execution  breakpoint,  set
              this to sizeof(long).

       config2 (since Linux 2.6.39)
              config2 is a further extension of the config1 field.

       branch_sample_type (since Linux 3.4)
              If  PERF_SAMPLE_BRANCH_STACK  is  enabled,  then  this  specifies  what branches to
              include in the branch record.

              The first part of the value is the privilege level, which is a combination  of  one
              of  the  values listed below.  If the user does not set privilege level explicitly,
              the kernel will use the event's privilege level.  Event and branch privilege levels
              do not have to match.

              PERF_SAMPLE_BRANCH_USER
                     Branch target is in user space.

              PERF_SAMPLE_BRANCH_KERNEL
                     Branch target is in kernel space.

              PERF_SAMPLE_BRANCH_HV
                     Branch target is in hypervisor.

              PERF_SAMPLE_BRANCH_PLM_ALL
                     A convenience value that is the three preceding values ORed together.

              In addition to the privilege value, at least one or more of the following bits must
              be set.

              PERF_SAMPLE_BRANCH_ANY
                     Any branch type.

              PERF_SAMPLE_BRANCH_ANY_CALL
                     Any call branch (includes direct calls, indirect calls, and far jumps).

              PERF_SAMPLE_BRANCH_IND_CALL
                     Indirect calls.

              PERF_SAMPLE_BRANCH_CALL (since Linux 4.4)
                     Direct calls.

              PERF_SAMPLE_BRANCH_ANY_RETURN
                     Any return branch.

              PERF_SAMPLE_BRANCH_IND_JUMP (since Linux 4.2)
                     Indirect jumps.

              PERF_SAMPLE_BRANCH_COND (since Linux 3.16)
                     Conditional branches.

              PERF_SAMPLE_BRANCH_ABORT_TX (since Linux 3.11)
                     Transactional memory aborts.

              PERF_SAMPLE_BRANCH_IN_TX (since Linux 3.11)
                     Branch in transactional memory transaction.

              PERF_SAMPLE_BRANCH_NO_TX (since Linux 3.11)
                     Branch      not       in       transactional       memory       transaction.
                     PERF_SAMPLE_BRANCH_CALL_STACK   (since  Linux  4.1)  Branch  is  part  of  a
                     hardware-generated call stack.  This requires  hardware  support,  currently
                     only found on Intel x86 Haswell or newer.

       sample_regs_user (since Linux 3.7)
              This bit mask defines the set of user CPU registers to dump on samples.  The layout
              of the register mask is architecture-specific and is described in the kernel header
              file arch/ARCH/include/uapi/asm/perf_regs.h.

       sample_stack_user (since Linux 3.7)
              This  defines  the  size  of  the  user  stack to dump if PERF_SAMPLE_STACK_USER is
              specified.

       clockid (since Linux 4.1)
              If use_clockid is set, then this field selects which internal Linux  timer  to  use
              for   timestamps.    The   available  timers  are  defined  in  linux/time.h,  with
              CLOCK_MONOTONIC, CLOCK_MONOTONIC_RAW, CLOCK_REALTIME, CLOCK_BOOTTIME, and CLOCK_TAI
              currently supported.

       aux_watermark (since Linux 4.1)
              This specifies how much data is required to trigger a PERF_RECORD_AUX sample.

       sample_max_stack (since Linux 4.8)
              When  sample_type  includes  PERF_SAMPLE_CALLCHAIN,  this  field specifies how many
              stack frames to report when generating the callchain.

   Reading results
       Once a perf_event_open() file descriptor has been opened, the values of the events can  be
       read from the file descriptor.  The values that are there are specified by the read_format
       field in the attr structure at open time.

       If you attempt to read into a buffer that is not big enough to hold the  data,  the  error
       ENOSPC results.

       Here is the layout of the data returned by a read:

       * If PERF_FORMAT_GROUP was specified to allow reading all events in a group at once:

             struct read_format {
                 u64 nr;            /* The number of events */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                 struct {
                     u64 value;     /* The value of the event */
                     u64 id;        /* if PERF_FORMAT_ID */
                 } values[nr];
             };

       * If PERF_FORMAT_GROUP was not specified:

             struct read_format {
                 u64 value;         /* The value of the event */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                 u64 id;            /* if PERF_FORMAT_ID */
             };

       The values read are as follows:

       nr     The  number of events in this file descriptor.  Available only if PERF_FORMAT_GROUP
              was specified.

       time_enabled, time_running
              Total time the event was enabled and running.  Normally these values are the  same.
              If  more  events  are  started,  then  available  counter  slots  on  the PMU, then
              multiplexing happens and events run only part of  the  time.   In  that  case,  the
              time_enabled  and  time  running values can be used to scale an estimated value for
              the count.

       value  An unsigned 64-bit value containing the counter result.

       id     A globally unique value for this particular event; only present  if  PERF_FORMAT_ID
              was specified in read_format.

   MMAP layout
       When  using  perf_event_open() in sampled mode, asynchronous events (like counter overflow
       or PROT_EXEC mmap tracking) are logged into a ring-buffer.  This  ring-buffer  is  created
       and accessed through mmap(2).

       The  mmap  size  should  be  1+2^n  pages, where the first page is a metadata page (struct
       perf_event_mmap_page) that contains various bits of information such as  where  the  ring-
       buffer head is.

       Before kernel 2.6.39, there is a bug that means you must allocate an mmap ring buffer when
       sampling even if you do not plan to access it.

       The structure of the first metadata mmap page is as follows:

           struct perf_event_mmap_page {
               __u32 version;        /* version number of this structure */
               __u32 compat_version; /* lowest version this is compat with */
               __u32 lock;           /* seqlock for synchronization */
               __u32 index;          /* hardware counter identifier */
               __s64 offset;         /* add to hardware counter value */
               __u64 time_enabled;   /* time event active */
               __u64 time_running;   /* time event on CPU */
               union {
                   __u64   capabilities;
                   struct {
                       __u64 cap_usr_time / cap_usr_rdpmc / cap_bit0 : 1,
                             cap_bit0_is_deprecated : 1,
                             cap_user_rdpmc         : 1,
                             cap_user_time          : 1,
                             cap_user_time_zero     : 1,
                   };
               };
               __u16 pmc_width;
               __u16 time_shift;
               __u32 time_mult;
               __u64 time_offset;
               __u64 __reserved[120];   /* Pad to 1 k */
               __u64 data_head;         /* head in the data section */
               __u64 data_tail;         /* user-space written tail */
               __u64 data_offset;       /* where the buffer starts */
               __u64 data_size;         /* data buffer size */
               __u64 aux_head;
               __u64 aux_tail;
               __u64 aux_offset;
               __u64 aux_size;

           }

       The following list describes the fields in  the  perf_event_mmap_page  structure  in  more
       detail:

       version
              Version number of this structure.

       compat_version
              The lowest version this is compatible with.

       lock   A seqlock for synchronization.

       index  A unique hardware counter identifier.

       offset When  using  rdpmc for reads this offset value must be added to the one returned by
              rdpmc to get the current total event count.

       time_enabled
              Time the event was active.

       time_running
              Time the event was running.

       cap_usr_time / cap_usr_rdpmc / cap_bit0 (since Linux 3.4)
              There was a bug in the definition of cap_usr_time and cap_usr_rdpmc from Linux  3.4
              until  Linux 3.11.  Both bits were defined to point to the same location, so it was
              impossible to know if cap_usr_time or cap_usr_rdpmc were actually set.

              Starting with Linux 3.12, these are renamed to cap_bit0  and  you  should  use  the
              cap_user_time and cap_user_rdpmc fields instead.

       cap_bit0_is_deprecated (since Linux 3.12)
              If  set,  this  bit  indicates  that  the  kernel  supports  the properly separated
              cap_user_time and cap_user_rdpmc bits.

              If not-set, it indicates an older kernel where cap_usr_time and  cap_usr_rdpmc  map
              to the same bit and thus both features should be used with caution.

       cap_user_rdpmc (since Linux 3.12)
              If  the  hardware  supports user-space read of performance counters without syscall
              (this is the "rdpmc" instruction on x86), then the following code can be used to do
              a read:

                  u32 seq, time_mult, time_shift, idx, width;
                  u64 count, enabled, running;
                  u64 cyc, time_offset;

                  do {
                      seq = pc->lock;
                      barrier();
                      enabled = pc->time_enabled;
                      running = pc->time_running;

                      if (pc->cap_usr_time && enabled != running) {
                          cyc = rdtsc();
                          time_offset = pc->time_offset;
                          time_mult   = pc->time_mult;
                          time_shift  = pc->time_shift;
                      }

                      idx = pc->index;
                      count = pc->offset;

                      if (pc->cap_usr_rdpmc && idx) {
                          width = pc->pmc_width;
                          count += rdpmc(idx - 1);
                      }

                      barrier();
                  } while (pc->lock != seq);

       cap_user_time (since Linux 3.12)
              This  bit  indicates the hardware has a constant, nonstop timestamp counter (TSC on
              x86).

       cap_user_time_zero (since Linux 3.12)
              Indicates the presence of time_zero which allows mapping timestamp  values  to  the
              hardware clock.

       pmc_width
              If  cap_usr_rdpmc,  this  field  provides the bit-width of the value read using the
              rdpmc or equivalent instruction.  This can be used to sign extend the result like:

                  pmc <<= 64 - pmc_width;
                  pmc >>= 64 - pmc_width; // signed shift right
                  count += pmc;

       time_shift, time_mult, time_offset

              If cap_usr_time, these  fields  can  be  used  to  compute  the  time  delta  since
              time_enabled (in nanoseconds) using rdtsc or similar.

                  u64 quot, rem;
                  u64 delta;
                  quot = (cyc >> time_shift);
                  rem = cyc & (((u64)1 << time_shift) - 1);
                  delta = time_offset + quot * time_mult +
                          ((rem * time_mult) >> time_shift);

              Where  time_offset,  time_mult,  time_shift,  and cyc are read in the seqcount loop
              described above.  This delta can then be added to enabled and possible running  (if
              idx), improving the scaling:

                  enabled += delta;
                  if (idx)
                      running += delta;
                  quot = count / running;
                  rem  = count % running;
                  count = quot * enabled + (rem * enabled) / running;

       time_zero (since Linux 3.12)

              If  cap_usr_time_zero is set, then the hardware clock (the TSC timestamp counter on
              x86) can be calculated from the time_zero, time_mult, and time_shift values:

                  time = timestamp - time_zero;
                  quot = time / time_mult;
                  rem  = time % time_mult;
                  cyc = (quot << time_shift) + (rem << time_shift) / time_mult;

              And vice versa:

                  quot = cyc >> time_shift;
                  rem  = cyc & (((u64)1 << time_shift) - 1);
                  timestamp = time_zero + quot * time_mult +
                      ((rem * time_mult) >> time_shift);

       data_head
              This points to the head of the data section.  The value continuously increases,  it
              does  not  wrap.   The  value  needs to be manually wrapped by the size of the mmap
              buffer before accessing the samples.

              On SMP-capable platforms, after reading the  data_head  value,  user  space  should
              issue an rmb().

       data_tail
              When the mapping is PROT_WRITE, the data_tail value should be written by user space
              to reflect the last read data.  In this case, the kernel will not overwrite  unread
              data.

       data_offset (since Linux 4.1)
              Contains  the  offset  of  the  location  in the mmap buffer where perf sample data
              begins.

       data_size (since Linux 4.1)
              Contains the size of the perf sample region within the mmap buffer.

       aux_head, aux_tail, aux_offset, aux_size (since Linux 4.1)
              The AUX region allows mmaping a separate  sample  buffer  for  high-bandwidth  data
              streams  (separate  from  the  main  perf  sample  buffer).   An example of a high-
              bandwidth stream is instruction  tracing  support,  as  is  found  in  newer  Intel
              processors.

              To set up an AUX area, first aux_offset needs to be set with an offset greater than
              data_offset+data_size and aux_size needs to be set to the desired buffer size.  The
              desired  offset and size must be page aligned, and the size must be a power of two.
              These values are then passed to mmap in order to map the AUX buffer.  Pages in  the
              AUX  buffer  are  included  as  part  of  the  RLIMIT_MEMLOCK  resource  limit (see
              setrlimit(2)), and also as part of the perf_event_mlock_kb allowance.

              By default, the AUX buffer will be truncated if it will not fit  in  the  available
              space  in the ring buffer.  If the AUX buffer is mapped as a read only buffer, then
              it will operate in ring buffer mode where old data will be overwritten by new.   In
              overwrite  mode, it might not be possible to infer where the new data began, and it
              is the consumer's job to disable measurement while reading to avoid  possible  data
              races.

              The  aux_head and aux_tail ring buffer pointers have the same behavior and ordering
              rules as the previous described data_head and data_tail.

       The following 2^n ring-buffer pages have the layout described below.

       If perf_event_attr.sample_id_all is set, then all event types will  have  the  sample_type
       selected  fields related to where/when (identity) an event took place (TID, TIME, ID, CPU,
       STREAM_ID) described in PERF_RECORD_SAMPLE below,  it  will  be  stashed  just  after  the
       perf_event_header  and the fields already present for the existing fields, that is, at the
       end of the payload.  This allows a newer perf.data file to  be  supported  by  older  perf
       tools, with the new optional fields being ignored.

       The mmap values start with a header:

           struct perf_event_header {
               __u32   type;
               __u16   misc;
               __u16   size;
           };

       Below,  we describe the perf_event_header fields in more detail.  For ease of reading, the
       fields with shorter descriptions are presented first.

       size   This indicates the size of the record.

       misc   The misc field contains additional information about the sample.

              The  CPU   mode   can   be   determined   from   this   value   by   masking   with
              PERF_RECORD_MISC_CPUMODE_MASK  and looking for one of the following (note these are
              not bit masks, only one can be set at a time):

              PERF_RECORD_MISC_CPUMODE_UNKNOWN
                     Unknown CPU mode.

              PERF_RECORD_MISC_KERNEL
                     Sample happened in the kernel.

              PERF_RECORD_MISC_USER
                     Sample happened in user code.

              PERF_RECORD_MISC_HYPERVISOR
                     Sample happened in the hypervisor.

              PERF_RECORD_MISC_GUEST_KERNEL (since Linux 2.6.35)
                     Sample happened in the guest kernel.

              PERF_RECORD_MISC_GUEST_USER  (since Linux 2.6.35)
                     Sample happened in guest user code.

              Since the following three statuses are generated by different  record  types,  they
              alias to the same bit:

              PERF_RECORD_MISC_MMAP_DATA (since Linux 3.10)
                     This  is  set  when  the mapping is not executable; otherwise the mapping is
                     executable.

              PERF_RECORD_MISC_COMM_EXEC (since Linux 3.16)
                     This is set for a PERF_RECORD_COMM record on kernels more recent than  Linux
                     3.16 if a process name change was caused by an exec(2) system call.

              PERF_RECORD_MISC_SWITCH_OUT (since Linux 4.3)
                     When   a   PERF_RECORD_SWITCH   or   PERF_RECORD_SWITCH_CPU_WIDE  record  is
                     generated, this bit indicates that the  context  switch  is  away  from  the
                     current process (instead of into the current process).

              In addition, the following bits can be set:

              PERF_RECORD_MISC_EXACT_IP
                     This  indicates  that  the  content  of  PERF_SAMPLE_IP points to the actual
                     instruction that triggered the event.  See also perf_event_attr.precise_ip.

              PERF_RECORD_MISC_EXT_RESERVED (since Linux 2.6.35)
                     This indicates there is extended data available (currently not used).

              PERF_RECORD_MISC_PROC_MAP_PARSE_TIMEOUT
                     This bit is not set by the kernel.  It is reserved for the  user-space  perf
                     utility  to  indicate that /proc/i[pid]/maps parsing was taking too long and
                     was stopped, and thus the mmap records may be truncated.

       type   The type value is one of the below.  The values in the corresponding  record  (that
              follows the header) depend on the type selected as shown.

              PERF_RECORD_MMAP
                  The  MMAP  events  record the PROT_EXEC mappings so that we can correlate user-
                  space IPs to code.  They have the following structure:

                      struct {
                          struct perf_event_header header;
                          u32    pid, tid;
                          u64    addr;
                          u64    len;
                          u64    pgoff;
                          char   filename[];
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated  memory.   len  is  the  length  of  the
                         allocated  memory.   pgoff  is  the page offset of the allocated memory.
                         filename is a string describing the backing of the allocated memory.

              PERF_RECORD_LOST
                  This record indicates when events are lost.

                      struct {
                          struct perf_event_header header;
                          u64    id;
                          u64    lost;
                          struct sample_id sample_id;
                      };

                  id     is the unique event ID for the samples that were lost.

                  lost   is the number of events that were lost.

              PERF_RECORD_COMM
                  This record indicates a change in the process name.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          char   comm[];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  comm   is a string containing the new name of the process.

              PERF_RECORD_EXIT
                  This record indicates a process exit event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, ppid;
                          u32    tid, ptid;
                          u64    time;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_THROTTLE, PERF_RECORD_UNTHROTTLE
                  This record indicates a throttle/unthrottle event.

                      struct {
                          struct perf_event_header header;
                          u64    time;
                          u64    id;
                          u64    stream_id;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_FORK
                  This record indicates a fork event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, ppid;
                          u32    tid, ptid;
                          u64    time;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_READ
                  This record indicates a read event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, tid;
                          struct read_format values;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_SAMPLE
                  This record indicates a sample.

                      struct {
                          struct perf_event_header header;
                          u64    sample_id;       /* if PERF_SAMPLE_IDENTIFIER */
                          u64    ip;              /* if PERF_SAMPLE_IP */
                          u32    pid, tid;        /* if PERF_SAMPLE_TID */
                          u64    time;            /* if PERF_SAMPLE_TIME */
                          u64    addr;            /* if PERF_SAMPLE_ADDR */
                          u64    id;              /* if PERF_SAMPLE_ID */
                          u64    stream_id;       /* if PERF_SAMPLE_STREAM_ID */
                          u32    cpu, res;        /* if PERF_SAMPLE_CPU */
                          u64    period;          /* if PERF_SAMPLE_PERIOD */
                          struct read_format v;   /* if PERF_SAMPLE_READ */
                          u64    nr;              /* if PERF_SAMPLE_CALLCHAIN */
                          u64    ips[nr];         /* if PERF_SAMPLE_CALLCHAIN */
                          u32    size;            /* if PERF_SAMPLE_RAW */
                          char  data[size];       /* if PERF_SAMPLE_RAW */
                          u64    bnr;             /* if PERF_SAMPLE_BRANCH_STACK */
                          struct perf_branch_entry lbr[bnr];
                                                  /* if PERF_SAMPLE_BRANCH_STACK */
                          u64    abi;             /* if PERF_SAMPLE_REGS_USER */
                          u64    regs[weight(mask)];
                                                  /* if PERF_SAMPLE_REGS_USER */
                          u64    size;            /* if PERF_SAMPLE_STACK_USER */
                          char   data[size];      /* if PERF_SAMPLE_STACK_USER */
                          u64    dyn_size;        /* if PERF_SAMPLE_STACK_USER &&
                                                     size != 0 */
                          u64    weight;          /* if PERF_SAMPLE_WEIGHT */
                          u64    data_src;        /* if PERF_SAMPLE_DATA_SRC */
                          u64    transaction;     /* if PERF_SAMPLE_TRANSACTION */
                          u64    abi;             /* if PERF_SAMPLE_REGS_INTR */
                          u64    regs[weight(mask)];
                                                  /* if PERF_SAMPLE_REGS_INTR */
                      };

                  sample_id
                      If PERF_SAMPLE_IDENTIFIER is enabled, a 64-bit unique ID is included.  This
                      is  a  duplication  of  the  PERF_SAMPLE_ID  id  value, but included at the
                      beginning of the sample so parsers can easily obtain the value.

                  ip  If PERF_SAMPLE_IP is enabled, then a 64-bit instruction  pointer  value  is
                      included.

                  pid, tid
                      If  PERF_SAMPLE_TID  is enabled, then a 32-bit process ID and 32-bit thread
                      ID are included.

                  time
                      If PERF_SAMPLE_TIME is enabled, then a 64-bit timestamp is included.   This
                      is  obtained  via  local_clock() which is a hardware timestamp if available
                      and the jiffies value if not.

                  addr
                      If PERF_SAMPLE_ADDR is enabled, then a 64-bit address is included.  This is
                      usually  the  address  of  a  tracepoint,  breakpoint,  or  software event;
                      otherwise the value is 0.

                  id  If PERF_SAMPLE_ID is enabled, a 64-bit unique ID is included.  If the event
                      is a member of an event group, the group leader ID is returned.  This ID is
                      the same as the one returned by PERF_FORMAT_ID.

                  stream_id
                      If PERF_SAMPLE_STREAM_ID is  enabled,  a  64-bit  unique  ID  is  included.
                      Unlike  PERF_SAMPLE_ID  the  actual  ID  is returned, not the group leader.
                      This ID is the same as the one returned by PERF_FORMAT_ID.

                  cpu, res
                      If PERF_SAMPLE_CPU is enabled, this is a 32-bit value indicating which  CPU
                      was being used, in addition to a reserved (unused) 32-bit value.

                  period
                      If  PERF_SAMPLE_PERIOD  is  enabled,  a 64-bit value indicating the current
                      sampling period is written.

                  v   If PERF_SAMPLE_READ is enabled, a structure of type read_format is included
                      which  has  values  for all events in the event group.  The values included
                      depend on the read_format value used at perf_event_open() time.

                  nr, ips[nr]
                      If PERF_SAMPLE_CALLCHAIN is enabled, then a 64-bit number is included which
                      indicates how many following 64-bit instruction pointers will follow.  This
                      is the current callchain.

                  size, data[size]
                      If PERF_SAMPLE_RAW is enabled, then  a  32-bit  value  indicating  size  is
                      included  followed  by an array of 8-bit values of length size.  The values
                      are padded with 0 to have 64-bit alignment.

                      This RAW record data is opaque with respect to the ABI.   The  ABI  doesn't
                      make any promises with respect to the stability of its content, it may vary
                      depending on event, hardware, and kernel version.

                  bnr, lbr[bnr]
                      If PERF_SAMPLE_BRANCH_STACK is enabled, then a 64-bit value indicating  the
                      number of records is included, followed by bnr perf_branch_entry structures
                      which each include the fields:

                      from   This indicates the source instruction (may not be a branch).

                      to     The branch target.

                      mispred
                             The branch target was mispredicted.

                      predicted
                             The branch target was predicted.

                      in_tx (since Linux 3.11)
                             The branch was in a transactional memory transaction.

                      abort (since Linux 3.11)
                             The branch was in an aborted transactional memory transaction.

                      cycles (since Linux 4.3)
                             This reports the number of cycles elapsed since the previous  branch
                             stack update.

                      The  entries are from most to least recent, so the first entry has the most
                      recent branch.

                      Support for mispred, predicted, and cycles is optional; if  not  supported,
                      those values will be 0.

                      The type of branches recorded is specified by the branch_sample_type field.

                  abi, regs[weight(mask)]
                      If  PERF_SAMPLE_REGS_USER  is  enabled,  then  the  user  CPU registers are
                      recorded.

                      The abi field is one of PERF_SAMPLE_REGS_ABI_NONE,  PERF_SAMPLE_REGS_ABI_32
                      or PERF_SAMPLE_REGS_ABI_64.

                      The  regs field is an array of the CPU registers that were specified by the
                      sample_regs_user attr field.  The number of values is the  number  of  bits
                      set in the sample_regs_user bit mask.

                  size, data[size], dyn_size
                      If  PERF_SAMPLE_STACK_USER  is  enabled,  then  the user stack is recorded.
                      This can be used to generate stack backtraces.  size is the size  requested
                      by  the user in sample_stack_user or else the maximum record size.  data is
                      the stack data (a raw dump of the memory pointed to by the stack pointer at
                      the time of sampling).  dyn_size is the amount of data actually dumped (can
                      be less than size).  Note that dyn_size is omitted if size is 0.

                  weight
                      If PERF_SAMPLE_WEIGHT is enabled, then  a  64-bit  value  provided  by  the
                      hardware  is recorded that indicates how costly the event was.  This allows
                      expensive events to stand out more clearly in profiles.

                  data_src
                      If PERF_SAMPLE_DATA_SRC is enabled, then a 64-bit value is recorded that is
                      made up of the following fields:

                      mem_op
                          Type of opcode, a bitwise combination of:

                          PERF_MEM_OP_NA          Not available
                          PERF_MEM_OP_LOAD        Load instruction
                          PERF_MEM_OP_STORE       Store instruction
                          PERF_MEM_OP_PFETCH      Prefetch
                          PERF_MEM_OP_EXEC        Executable code

                      mem_lvl
                          Memory  hierarchy  level  hit  or  miss,  a  bitwise combination of the
                          following, shifted left by PERF_MEM_LVL_SHIFT:

                          PERF_MEM_LVL_NA         Not available
                          PERF_MEM_LVL_HIT        Hit
                          PERF_MEM_LVL_MISS       Miss
                          PERF_MEM_LVL_L1         Level 1 cache
                          PERF_MEM_LVL_LFB        Line fill buffer
                          PERF_MEM_LVL_L2         Level 2 cache
                          PERF_MEM_LVL_L3         Level 3 cache
                          PERF_MEM_LVL_LOC_RAM    Local DRAM
                          PERF_MEM_LVL_REM_RAM1   Remote DRAM 1 hop
                          PERF_MEM_LVL_REM_RAM2   Remote DRAM 2 hops
                          PERF_MEM_LVL_REM_CCE1   Remote cache 1 hop
                          PERF_MEM_LVL_REM_CCE2   Remote cache 2 hops
                          PERF_MEM_LVL_IO         I/O memory
                          PERF_MEM_LVL_UNC        Uncached memory

                      mem_snoop
                          Snoop mode, a bitwise combination of the  following,  shifted  left  by
                          PERF_MEM_SNOOP_SHIFT:

                          PERF_MEM_SNOOP_NA       Not available
                          PERF_MEM_SNOOP_NONE     No snoop
                          PERF_MEM_SNOOP_HIT      Snoop hit
                          PERF_MEM_SNOOP_MISS     Snoop miss
                          PERF_MEM_SNOOP_HITM     Snoop hit modified

                      mem_lock
                          Lock  instruction, a bitwise combination of the following, shifted left
                          by PERF_MEM_LOCK_SHIFT:

                          PERF_MEM_LOCK_NA        Not available
                          PERF_MEM_LOCK_LOCKED    Locked transaction

                      mem_dtlb
                          TLB access hit or miss, a bitwise combination of the following, shifted
                          left by PERF_MEM_TLB_SHIFT:

                          PERF_MEM_TLB_NA         Not available
                          PERF_MEM_TLB_HIT        Hit
                          PERF_MEM_TLB_MISS       Miss
                          PERF_MEM_TLB_L1         Level 1 TLB
                          PERF_MEM_TLB_L2         Level 2 TLB
                          PERF_MEM_TLB_WK         Hardware walker
                          PERF_MEM_TLB_OS         OS fault handler

                  transaction
                      If the PERF_SAMPLE_TRANSACTION flag is set, then a 64-bit field is recorded
                      describing the sources of any transactional memory aborts.

                      The field is a bitwise combination of the following values:

                      PERF_TXN_ELISION
                             Abort from an elision type transaction (Intel-CPU-specific).

                      PERF_TXN_TRANSACTION
                             Abort from a generic transaction.

                      PERF_TXN_SYNC
                             Synchronous abort (related to the reported instruction).

                      PERF_TXN_ASYNC
                             Asynchronous abort (not related to the reported instruction).

                      PERF_TXN_RETRY
                             Retryable abort (retrying the transaction may have succeeded).

                      PERF_TXN_CONFLICT
                             Abort due to memory conflicts with other threads.

                      PERF_TXN_CAPACITY_WRITE
                             Abort due to write capacity overflow.

                      PERF_TXN_CAPACITY_READ
                             Abort due to read capacity overflow.

                      In addition, a user-specified abort code can be obtained from the  high  32
                      bits  of  the  field  by shifting right by PERF_TXN_ABORT_SHIFT and masking
                      with the value PERF_TXN_ABORT_MASK.

                  abi, regs[weight(mask)]
                      If PERF_SAMPLE_REGS_INTR is  enabled,  then  the  user  CPU  registers  are
                      recorded.

                      The abi field is one of PERF_SAMPLE_REGS_ABI_NONE, PERF_SAMPLE_REGS_ABI_32,
                      or PERF_SAMPLE_REGS_ABI_64.

                      The regs field is an array of the CPU registers that were specified by  the
                      sample_regs_intr  attr  field.   The number of values is the number of bits
                      set in the sample_regs_intr bit mask.

              PERF_RECORD_MMAP2
                  This record includes extended information on mmap(2) calls returning executable
                  mappings.   The  format  is similar to that of the PERF_RECORD_MMAP record, but
                  includes extra values that allow uniquely identifying shared mappings.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          u64    addr;
                          u64    len;
                          u64    pgoff;
                          u32    maj;
                          u32    min;
                          u64    ino;
                          u64    ino_generation;
                          u32    prot;
                          u32    flags;
                          char   filename[];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated memory.

                  len    is the length of the allocated memory.

                  pgoff  is the page offset of the allocated memory.

                  maj    is the major ID of the underlying device.

                  min    is the minor ID of the underlying device.

                  ino    is the inode number.

                  ino_generation
                         is the inode generation.

                  prot   is the protection information.

                  flags  is the flags information.

                  filename
                         is a string describing the backing of the allocated memory.

              PERF_RECORD_AUX (since Linux 4.1)

                  This record reports that new data is  available  in  the  separate  AUX  buffer
                  region.

                      struct {
                          struct perf_event_header header;
                          u64    aux_offset;
                          u64    aux_size;
                          u64    flags;
                          struct sample_id sample_id;
                      };

                  aux_offset
                         offset in the AUX mmap region where the new data begins.

                  aux_size
                         size of the data made available.

                  flags  describes the AUX update.

                         PERF_AUX_FLAG_TRUNCATED
                                if set, then the data returned was truncated to fit the available
                                buffer size.

                         PERF_AUX_FLAG_OVERWRITE
                                if set, then the data returned has overwritten previous data.

              PERF_RECORD_ITRACE_START (since Linux 4.1)

                  This record indicates which process has initiated an instruction  trace  event,
                  allowing  tools  to  properly  correlate  the  instruction addresses in the AUX
                  buffer with the proper executable.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                      };

                  pid    process ID of the thread starting an instruction trace.

                  tid    thread ID of the thread starting an instruction trace.

              PERF_RECORD_LOST_SAMPLES (since Linux 4.2)

                  When using hardware sampling (such as Intel PEBS) this  record  indicates  some
                  number of samples that may have been lost.

                      struct {
                          struct perf_event_header header;
                          u64    lost;
                          struct sample_id sample_id;
                      };

                  lost   the number of potentially lost samples.

              PERF_RECORD_SWITCH (since Linux 4.3)

                  This    record    indicates    a    context    switch    has   happened.    The
                  PERF_RECORD_MISC_SWITCH_OUT bit in the misc field indicates whether  it  was  a
                  context switch into or away from the current process.

                      struct {
                          struct perf_event_header header;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_SWITCH_CPU_WIDE (since Linux 4.3)

                  As with PERF_RECORD_SWITCH this record indicates a context switch has happened,
                  but it only occurs when sampling  in  CPU-wide  mode  and  provides  additional
                  information     on     the     process    being    switched    to/from.     The
                  PERF_RECORD_MISC_SWITCH_OUT bit in the misc field indicates whether  it  was  a
                  context switch into or away from the current process.

                      struct {
                          struct perf_event_header header;
                          u32 next_prev_pid;
                          u32 next_prev_tid;
                          struct sample_id sample_id;
                      };

                  next_prev_pid
                         The  process  ID of the previous (if switching in) or next (if switching
                         out) process on the CPU.

                  next_prev_tid
                         The thread ID of the previous (if switching in) or  next  (if  switching
                         out) thread on the CPU.

   Overflow handling
       Events can be set to notify when a threshold is crossed, indicating an overflow.  Overflow
       conditions can  be  captured  by  monitoring  the  event  file  descriptor  with  poll(2),
       select(2),  or epoll(7).  Alternatively, the overflow events can be captured via sa signal
       handler, by enabling I/O signaling on the file  descriptor;  see  the  discussion  of  the
       F_SETOWN and F_SETSIG operations in fcntl(2).

       Overflows are generated only by sampling events (sample_period must have a nonzero value).

       There are two ways to generate overflow notifications.

       The  first  is  to  set  a  wakeup_events or wakeup_watermark value that will trigger if a
       certain number of samples or bytes have been written to the mmap  ring  buffer.   In  this
       case, POLL_IN is indicated.

       The other way is by use of the PERF_EVENT_IOC_REFRESH ioctl.  This ioctl adds to a counter
       that decrements each time the event overflows.  When nonzero, POLL_IN  is  indicated,  but
       once the counter reaches 0 POLL_HUP is indicated and the underlying event is disabled.

       Refreshing an event group leader refreshes all siblings and refreshing with a parameter of
       0 currently enables infinite refreshes; these behaviors are unsupported and should not  be
       relied on.

       Starting  with  Linux 3.18, POLL_HUP is indicated if the event being monitored is attached
       to a different process and that process exits.

   rdpmc instruction
       Starting with Linux 3.4 on x86, you can use the rdpmc instruction to get low-latency reads
       without  having to enter the kernel.  Note that using rdpmc is not necessarily faster than
       other methods for reading event values.

       Support for this  can  be  detected  with  the  cap_usr_rdpmc  field  in  the  mmap  page;
       documentation on how to calculate event values can be found in that section.

       Originally, when rdpmc support was enabled, any process (not just ones with an active perf
       event) could use the rdpmc instruction to access the counters.  Starting with  Linux  4.0,
       rdpmc support is only allowed if an event is currently enabled in a process's context.  To
       restore the old behavior, write the value 2 to /sys/devices/cpu/rdpmc.

   perf_event ioctl calls
       Various ioctls act on perf_event_open() file descriptors:

       PERF_EVENT_IOC_ENABLE
              This enables the individual event or event group specified by the  file  descriptor
              argument.

              If  the  PERF_IOC_FLAG_GROUP bit is set in the ioctl argument, then all events in a
              group are enabled, even if the event specified is not the  group  leader  (but  see
              BUGS).

       PERF_EVENT_IOC_DISABLE
              This  disables  the  individual  counter  or  event  group  specified  by  the file
              descriptor argument.

              Enabling or disabling the leader of a group enables or disables the  entire  group;
              that is, while the group leader is disabled, none of the counters in the group will
              count.  Enabling or disabling a member of a group other  than  the  leader  affects
              only  that  counter;  disabling  a  non-leader stops that counter from counting but
              doesn't affect any other counter.

              If the PERF_IOC_FLAG_GROUP bit is set in the ioctl argument, then all events  in  a
              group  are  disabled,  even if the event specified is not the group leader (but see
              BUGS).

       PERF_EVENT_IOC_REFRESH
              Non-inherited overflow counters can use this to enable a counter for  a  number  of
              overflows  specified by the argument, after which it is disabled.  Subsequent calls
              of  this  ioctl  add  the  argument  value  to  the  current  count.   An  overflow
              notification  with POLL_IN set will happen on each overflow until the count reaches
              0; when that happens a notification with POLL_HUP set is  sent  and  the  event  is
              disabled.  Using an argument of 0 is considered undefined behavior.

       PERF_EVENT_IOC_RESET
              Reset  the  event  count  specified  by the file descriptor argument to zero.  This
              resets only the counts; there is no way to reset the multiplexing  time_enabled  or
              time_running values.

              If  the  PERF_IOC_FLAG_GROUP bit is set in the ioctl argument, then all events in a
              group are reset, even if the event specified is  not  the  group  leader  (but  see
              BUGS).

       PERF_EVENT_IOC_PERIOD
              This updates the overflow period for the event.

              Since  Linux  3.7 (on ARM) and Linux 3.14 (all other architectures), the new period
              takes effect immediately.  On older kernels, the new period  did  not  take  effect
              until after the next overflow.

              The argument is a pointer to a 64-bit value containing the desired new period.

              Prior to Linux 2.6.36, this ioctl always failed due to a bug in the kernel.

       PERF_EVENT_IOC_SET_OUTPUT
              This  tells  the  kernel  to  report  event  notifications  to  the  specified file
              descriptor rather than the default one.  The file descriptors must all  be  on  the
              same CPU.

              The  argument  specifies  the  desired  file  descriptor, or -1 if output should be
              ignored.

       PERF_EVENT_IOC_SET_FILTER (since Linux 2.6.33)
              This adds an ftrace filter to this event.

              The argument is a pointer to the desired ftrace filter.

       PERF_EVENT_IOC_ID (since Linux 3.12)
              This returns the event ID value for the given event file descriptor.

              The argument is a pointer to a 64-bit unsigned integer to hold the result.

       PERF_EVENT_IOC_SET_BPF (since Linux 4.1)
              This allows attaching a Berkeley Packet Filter (BPF) program to an existing  kprobe
              tracepoint event.  You need CAP_SYS_ADMIN privileges to use this ioctl.

              The argument is a BPF program file descriptor that was created by a previous bpf(2)
              system call.

   Using prctl(2)
       A process can enable or disable all the event groups that are attached  to  it  using  the
       prctl(2)  PR_TASK_PERF_EVENTS_ENABLE  and  PR_TASK_PERF_EVENTS_DISABLE  operations.   This
       applies to all counters on the calling process, whether created  by  this  process  or  by
       another,  and  does  not  affect  any  counters  that  this  process  has created on other
       processes.  It enables or disables only the group leaders, not any other  members  in  the
       groups.

   perf_event related configuration files
       Files in /proc/sys/kernel/

           /proc/sys/kernel/perf_event_paranoid
                  The  perf_event_paranoid  file can be set to restrict access to the performance
                  counters.

                  2   allow only user-space measurements (default since Linux 4.6).
                  1   allow both kernel and user measurements (default before Linux 4.6).
                  0   allow access to CPU-specific data but not raw tracepoint samples.
                  -1  no restrictions.

                  The existence of the  perf_event_paranoid  file  is  the  official  method  for
                  determining if a kernel supports perf_event_open().

           /proc/sys/kernel/perf_event_max_sample_rate
                  This  sets  the  maximum sample rate.  Setting this too high can allow users to
                  sample at a rate that impacts overall machine performance and potentially  lock
                  up the machine.  The default value is 100000 (samples per second).

           /proc/sys/kernel/perf_event_max_stack
                  This  file  sets  the  maximum  depth  of  stack  frame  entries  reported when
                  generating a call trace.

           /proc/sys/kernel/perf_event_mlock_kb
                  Maximum number of pages an unprivileged user can mlock(2).  The default is  516
                  (kB).

       Files in /sys/bus/event_source/devices/

           Since Linux 2.6.34, the kernel supports having multiple PMUs available for monitoring.
           Information   on   how   to    program    these    PMUs    can    be    found    under
           /sys/bus/event_source/devices/.  Each subdirectory corresponds to a different PMU.

           /sys/bus/event_source/devices/*/type (since Linux 2.6.38)
                  This  contains an integer that can be used in the type field of perf_event_attr
                  to indicate that you wish to use this PMU.

           /sys/bus/event_source/devices/cpu/rdpmc (since Linux 3.4)
                  If this file is 1, then direct user-space access  to  the  performance  counter
                  registers  is  allowed  via  the  rdpmc  instruction.   This can be disabled by
                  echoing 0 to the file.

                  As of Linux 4.0 the behavior has changed, so that 1 now means only allow access
                  to  processes  with active perf events, with 2 indicating the old allow-anyone-
                  access behavior.

           /sys/bus/event_source/devices/*/format/ (since Linux 3.4)
                  This subdirectory contains information on the  architecture-specific  subfields
                  available  for  programming  the  various  config fields in the perf_event_attr
                  struct.

                  The content of each file is the name of the config field, followed by a  colon,
                  followed  by  a series of integer bit ranges separated by commas.  For example,
                  the file event may contain the value  config1:1,6-10,44  which  indicates  that
                  event    is   an   attribute   that   occupies   bits   1,6–10,   and   44   of
                  perf_event_attr::config1.

           /sys/bus/event_source/devices/*/events/ (since Linux 3.4)
                  This subdirectory contains files with  predefined  events.   The  contents  are
                  strings describing the event settings expressed in terms of the fields found in
                  the previously  mentioned  ./format/  directory.   These  are  not  necessarily
                  complete lists of all events supported by a PMU, but usually a subset of events
                  deemed useful or interesting.

                  The content of each file is a list of  attribute  names  separated  by  commas.
                  Each  entry  has  an  optional  value  (either hex or decimal).  If no value is
                  specified, then it is assumed to be a single-bit field with a value of  1.   An
                  example entry may look like this: event=0x2,inv,ldlat=3.

           /sys/bus/event_source/devices/*/uevent
                  This file is the standard kernel device interface for injecting hotplug events.

           /sys/bus/event_source/devices/*/cpumask (since Linux 3.7)
                  The  cpumask  file  contains a comma-separated list of integers that indicate a
                  representative CPU number for each socket (package) on the  motherboard.   This
                  is  needed  when setting up uncore or northbridge events, as those PMUs present
                  socket-wide events.

RETURN VALUE

       perf_event_open() returns the new file descriptor, or -1 if an error  occurred  (in  which
       case, errno is set appropriately).

ERRORS

       The  errors  returned  by  perf_event_open()  can  be  inconsistent,  and  may vary across
       processor architectures and performance monitoring units.

       E2BIG  Returned  if  the  perf_event_attr  size  value  is   too   small   (smaller   than
              PERF_ATTR_SIZE_VER0),  too  big  (larger  than  the  page size), or larger than the
              kernel supports and the extra bytes are not zero.   When  E2BIG  is  returned,  the
              perf_event_attr  size  field  is  overwritten  by  the kernel to be the size of the
              structure it was expecting.

       EACCES Returned when the requested event requires CAP_SYS_ADMIN  permissions  (or  a  more
              permissive  perf_event  paranoid setting).  Some common cases where an unprivileged
              process may encounter this error: attaching to a process owned by a different user;
              monitoring  all processes on a given CPU (i.e., specifying the pid argument as -1);
              and not setting exclude_kernel when the paranoid setting requires it.

       EBADF  Returned if the group_fd file descriptor is not valid, or, if  PERF_FLAG_PID_CGROUP
              is set, the cgroup file descriptor in pid is not valid.

       EBUSY (since Linux 4.1)
              Returned if another event already has exclusive access to the PMU.

       EFAULT Returned if the attr pointer points at an invalid memory address.

       EINVAL Returned  if  the  specified event is invalid.  There are many possible reasons for
              this.  A not-exhaustive list: sample_freq is higher than the maximum  setting;  the
              cpu  to  monitor does not exist; read_format is out of range; sample_type is out of
              range; the flags value is out of range; exclusive or pinned set and  the  event  is
              not  a group leader; the event config values are out of range or set reserved bits;
              the generic event selected is not supported; or there is not enough room to add the
              selected event.

       EMFILE Each  opened  event  uses  one  file  descriptor.   If a large number of events are
              opened, the per-process limit on the  number  of  open  file  descriptors  will  be
              reached, and no more events can be created.

       ENODEV Returned when the event involves a feature not supported by the current CPU.

       ENOENT Returned  if  the  type setting is not valid.  This error is also returned for some
              unsupported generic events.

       ENOSPC Prior to Linux 3.3, if there  was  not  enough  room  for  the  event,  ENOSPC  was
              returned.   In  Linux 3.3, this was changed to EINVAL.  ENOSPC is still returned if
              you try to add more breakpoint events than supported by the hardware.

       ENOSYS Returned if PERF_SAMPLE_STACK_USER is set in sample_type and it is not supported by
              hardware.

       EOPNOTSUPP
              Returned  if  an event requiring a specific hardware feature is requested but there
              is no hardware support.  This includes requesting low-skid events if not supported,
              branch  tracing  if it is not available, sampling if no PMU interrupt is available,
              and branch stacks for software events.

       EOVERFLOW (since Linux 4.8)
              Returned if PERF_SAMPLE_CALLCHAIN is requested and sample_max_stack is larger  than
              the maximum specified in /proc/sys/kernel/perf_event_max_stack.

       EPERM  Returned  on  many  (but  not  all)  architectures  when an unsupported exclude_hv,
              exclude_idle, exclude_user, or exclude_kernel setting is specified.

              It can also happen, as with EACCES, when the requested event requires CAP_SYS_ADMIN
              permissions  (or  a  more  permissive  perf_event paranoid setting).  This includes
              setting a breakpoint on a kernel address, and (since Linux 3.13) setting  a  kernel
              function-trace tracepoint.

       ESRCH  Returned if attempting to attach to a process that does not exist.

VERSION

       perf_event_open()  was  introduced in Linux 2.6.31 but was called perf_counter_open().  It
       was renamed in Linux 2.6.32.

CONFORMING TO

       This perf_event_open() system call Linux-specific and  should  not  be  used  in  programs
       intended to be portable.

NOTES

       Glibc  does not provide a wrapper for this system call; call it using syscall(2).  See the
       example below.

       The official way of knowing if perf_event_open() support is enabled is  checking  for  the
       existence of the file /proc/sys/kernel/perf_event_paranoid.

BUGS

       The  F_SETOWN_EX option to fcntl(2) is needed to properly get overflow signals in threads.
       This was introduced in Linux 2.6.32.

       Prior to Linux 2.6.33 (at least for x86), the kernel did not  check  if  events  could  be
       scheduled  together  until  read  time.   The same happens on all known kernels if the NMI
       watchdog is enabled.  This means to see if a  given  set  of  events  works  you  have  to
       perf_event_open(),  start,  then  read  before  you  know  for  sure  you  can  get  valid
       measurements.

       Prior to Linux 2.6.34, event constraints were not enforced by the kernel.  In  that  case,
       some  events would silently return "0" if the kernel scheduled them in an improper counter
       slot.

       Prior to Linux 2.6.34, there was a bug when multiplexing where the wrong results could  be
       returned.

       Kernels  from  Linux  2.6.35  to Linux 2.6.39 can quickly crash the kernel if "inherit" is
       enabled and many threads are started.

       Prior to Linux 2.6.35, PERF_FORMAT_GROUP did not work with attached processes.

       There is a bug in the kernel code between Linux 2.6.36 and  Linux  3.0  that  ignores  the
       "watermark"  field  and  acts  as  if a wakeup_event was chosen if the union has a nonzero
       value in it.

       From Linux 2.6.31 to Linux 3.4, the PERF_IOC_FLAG_GROUP  ioctl  argument  was  broken  and
       would  repeatedly  operate on the event specified rather than iterating across all sibling
       events in a group.

       From Linux 3.4 to Linux 3.11, the mmap cap_usr_rdpmc and cap_usr_time bits mapped  to  the
       same  location.   Code  should  migrate to the new cap_user_rdpmc and cap_user_time fields
       instead.

       Always double-check your results!  Various generalized events have had wrong values.   For
       example, retired branches measured the wrong thing on AMD machines until Linux 2.6.35.

EXAMPLE

       The  following  is  a short example that measures the total instruction count of a call to
       printf(3).

       #include <stdlib.h>
       #include <stdio.h>
       #include <unistd.h>
       #include <string.h>
       #include <sys/ioctl.h>
       #include <linux/perf_event.h>
       #include <asm/unistd.h>

       static long
       perf_event_open(struct perf_event_attr *hw_event, pid_t pid,
                       int cpu, int group_fd, unsigned long flags)
       {
           int ret;

           ret = syscall(__NR_perf_event_open, hw_event, pid, cpu,
                          group_fd, flags);
           return ret;
       }

       int
       main(int argc, char **argv)
       {
           struct perf_event_attr pe;
           long long count;
           int fd;

           memset(&pe, 0, sizeof(struct perf_event_attr));
           pe.type = PERF_TYPE_HARDWARE;
           pe.size = sizeof(struct perf_event_attr);
           pe.config = PERF_COUNT_HW_INSTRUCTIONS;
           pe.disabled = 1;
           pe.exclude_kernel = 1;
           pe.exclude_hv = 1;

           fd = perf_event_open(&pe, 0, -1, -1, 0);
           if (fd == -1) {
              fprintf(stderr, "Error opening leader %llx\n", pe.config);
              exit(EXIT_FAILURE);
           }

           ioctl(fd, PERF_EVENT_IOC_RESET, 0);
           ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

           printf("Measuring instruction count for this printf\n");

           ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
           read(fd, &count, sizeof(long long));

           printf("Used %lld instructions\n", count);

           close(fd);
       }

SEE ALSO

       fcntl(2), mmap(2), open(2), prctl(2), read(2)

COLOPHON

       This page is part of release 4.13 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.