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       perf_event_open - set up performance monitoring


       #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.


       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).

       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).

              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 kprobe_func;      /* for perf_kprobe */
                   __u64 uprobe_path;      /* for perf_uprobe */
                   __u64 config1;          /* extension of config */

               union {
                   __u64 bp_len;           /* breakpoint length */
                   __u64 kprobe_addr;      /* with kprobe_func == NULL */
                   __u64 probe_offset;     /* for perf_[k,u]probe */
                   __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:

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

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

                     This   indicates   a   tracepoint   provided   by   the   kernel  tracepoint

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

                     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.

              kprobe and uprobe (since Linux 4.17)
                     These two dynamic PMUs create a kprobe/uprobe and  attach  it  to  the  file
                     descriptor   generated   by  perf_event_open.   The  kprobe/uprobe  will  be
                     destroyed  on  the  destruction  of  the  file   descriptor.    See   fields
                     kprobe_func, uprobe_path, kprobe_addr, and probe_offset for more details.

       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

              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

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

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

                          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.

                          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.

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

                          Mispredicted branch instructions.

                          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:

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

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

                          This reports the number of page faults.

                          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.

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

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

                          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

                   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:

                             for measuring Level 1 Data Cache

                             for measuring Level 1 Instruction Cache

                             for measuring Last-Level Cache

                             for measuring the Data TLB

                             for measuring the Instruction TLB

                             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:

                             for read accesses

                             for write accesses

                             for prefetch accesses

                  and perf_hw_cache_op_result_id is one of:

                             to measure accesses

                             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.

              If   type   is   kprobe   or   uprobe,   set   retprobe   (bit  0  of  config,  see
              /sys/bus/event_source/devices/[k,u]probe/format/retprobe) for  kretprobe/uretprobe.
              See  fields  kprobe_func,  uprobe_path,  kprobe_addr,  and  probe_offset  for  more

       kprobe_func, uprobe_path, kprobe_addr, and probe_offset
              These fields describe the kprobe/uprobe for dynamic PMUs kprobe  and  uprobe.   For
              kprobe:  use kprobe_func and probe_offset, or use kprobe_addr and leave kprobe_func
              as NULL.  For uprobe: use uprobe_path and probe_offset.

       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.

              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.

                     Records instruction pointer.

                     Records the process and thread IDs.

                     Records a timestamp.

                     Records an address, if applicable.

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

                     Records the callchain (stack backtrace).

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

                     Records CPU number.

                     Records the current sampling period.

                     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.

                     Records additional data, if  applicable.   Usually  returned  by  tracepoint

              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

              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.

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

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

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

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

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

              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

              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

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

       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.

              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).

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

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

              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.

              If set, don't count when the CPU is running the idle task.  While you can currently
              enable this for any event type, it is ignored for all but software events.

       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.

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

              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.

              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

       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 */

       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

       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

              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:

                     No breakpoint.

                     Count when we read the memory location.

                     Count when we write the memory location.

                     Count when we read or write the memory location.

                     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
              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.

                     Branch target is in user space.

                     Branch target is in kernel space.

                     Branch target is in hypervisor.

                     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.

                     Any branch type.

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

                     Indirect calls.

              PERF_SAMPLE_BRANCH_CALL (since Linux 4.4)
                     Direct calls.

                     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

       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
              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.
              Multiplexing happens if the number of events is more than the number  of  available
              PMU  counter  slots.   In  that  case  the events run only part of the time and 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

              Version number of this structure.

              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 the event was active.

              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;
                      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);

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

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

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

              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);

              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().

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

       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

              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

              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 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):

                     Unknown CPU mode.

                     Sample happened in the kernel.

                     Sample happened in user code.

                     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

              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:

                     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).

                     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.

                  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.

                  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.

                  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.

                  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;

                  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;

                  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;

                  This record indicates a read event.

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

                  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 */

                      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

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

                      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.

                      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.

                      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.

                      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.

                             The branch target was mispredicted.

                             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

                      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.

                      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.

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

                          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

                          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

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

                          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

                          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

                          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

                      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:

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

                             Abort from a generic transaction.

                             Synchronous abort (related to the reported instruction).

                             Asynchronous abort (not related to the reported instruction).

                             Retryable abort (retrying the transaction may have succeeded).

                             Abort due to memory conflicts with other threads.

                             Abort due to write capacity overflow.

                             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

                      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.

                  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.

                         is the inode generation.

                  prot   is the protection information.

                  flags  is the flags information.

                         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

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

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

                         size of the data made available.

                  flags  describes the AUX update.

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

                                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;

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

                         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:

              This  enables  the individual event or event group specified by the file descriptor

              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

              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

              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.

              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

              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.

              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

       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.

       PERF_EVENT_IOC_PAUSE_OUTPUT (since Linux 4.7)
              This  allows  pausing  and  resuming the event's ring-buffer.  A paused ring-buffer
              does not prevent generation of samples, but simply discards  them.   The  discarded
              samples  are  considered  lost, and cause a PERF_RECORD_LOST sample to be generated
              when possible.  An overflow signal may still be triggered by the  discarded  sample
              even though the ring-buffer remains empty.

              The  argument  is  an  unsigned  32-bit  integer.  A nonzero value pauses the ring-
              buffer, while a zero value resumes the ring-buffer.

       PERF_EVENT_MODIFY_ATTRIBUTES (since Linux 4.17)
              This allows modifying an  existing  event  without  the  overhead  of  closing  and
              reopening a new event.  Currently this is supported only for breakpoint events.

              The  argument  is  a  pointer to a perf_event_attr structure containing the updated
              event settings.

       PERF_EVENT_IOC_QUERY_BPF (since Linux 4.16)
              This allows querying which Berkeley Packet Filter (BPF) programs are attached to an
              existing kprobe tracepoint.  You can only attach one BPF program per event, but you
              can have multiple events attached to a tracepoint.   Querying  this  value  on  one
              tracepoint  event  returns the id of all BPF programs in all events attached to the
              tracepoint.  You need CAP_SYS_ADMIN privileges to use this ioctl.

              The argument is a pointer to a structure
                  struct perf_event_query_bpf {
                      __u32    ids_len;
                      __u32    prog_cnt;
                      __u32    ids[0];

              The ids_len field indicates the number of ids that can  fit  in  the  provided  ids
              array.   The  prog_cnt value is filled in by the kernel with the number of attached
              BPF programs.  The ids array is filled with the id of each  attached  BPF  program.
              If  there are more programs than will fit in the array, then the kernel will return
              ENOSPC and ids_len will indicate the number of program IDs that  were  successfully

   Using prctl(2)
       A  process  can  enable  or  disable  all  currently  open event groups using the prctl(2)
       PR_TASK_PERF_EVENTS_ENABLE and PR_TASK_PERF_EVENTS_DISABLE operations.  This applies  only
       to  events  created locally by the calling process.  This does not apply to events created
       by other processes attached to the calling process  or  inherited  events  from  a  parent
       process.   Only  group  leaders  are  enabled  and  disabled, not any other members of the

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

                  The perf_event_paranoid file can be set to restrict access to  the  performance

                  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().

                  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).

                  This file  sets  the  maximum  depth  of  stack  frame  entries  reported  when
                  generating a call trace.

                  Maximum  number of pages an unprivileged user can mlock(2).  The default is 516

       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

                  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

           /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.

                  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.


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


       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.

       EINTR  Returned when trying to mix perf and ftrace handling for a uprobe.

       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

              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.


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


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


       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.


       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

       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

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

       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

       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.


       The following is a short example that measures the total instruction count of  a  call  to

       #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;

       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);

           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);



       perf(1), fcntl(2), mmap(2), open(2), prctl(2), read(2)

       Documentation/admin-guide/perf-security.rst in the kernel source tree


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