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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

       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.

       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

       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 */

                     __reserved_1   : 38;

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

               __u32     bp_type;          /* breakpoint type */

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

               union {
                   __u64 bp_len;           /* breakpoint length */
                   __u64 config2;          /* extension of config1 */
               __u64 branch_sample_type;   /* enum perf_branch_sample_type */
               __u64 sample_regs_user;     /* user regs to dump on samples */
               __u32 sample_stack_user;    /* size of stack to dump on
                                              samples */
               __s32 clockid;              /* clock to use for time fields */
               __u64 sample_regs_intr;     /* regs to dump on samples */
               __u32 aux_watermark;        /* aux bytes before wakeup */
               __u32 __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

                     This  indicates  a  tracepoint  provided  by  the  kernel
                     tracepoint infrastructure.

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

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

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

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

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

              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_watermak in Linux 4.1.

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

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

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

                          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

                          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

                   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

              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.

       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

              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

                     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

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

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

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

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

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

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

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

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

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

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

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

                     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

              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

                     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_formats,
              such as PERF_FORMAT_GROUP.

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

              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

              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 idle.

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

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

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

              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 values of this 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

       mmap_data (since Linux 2.6.36)
              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 only.

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

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

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

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

       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.

       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

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

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

       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)
              bp_addr 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
              3.3 and later kernels.

       bp_len (since Linux 2.6.33)
              bp_len is the length of the breakpoint being measured if type is
              PERF_TYPE_BREAKPOINT.     Options    are    HW_BREAKPOINT_LEN_1,
              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 following values.  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.

                     Any return branch.

                     Indirect calls.

              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    described    in     the     kernel     header

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

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

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

   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 ENOSPC is returned

       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.  Only available if
              PERF_FORMAT_GROUP was specified.

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

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

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

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

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

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

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

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


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

              Version 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 x86).

       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

                  u64 quot, rem;
                  u64 delta;
                  quot = (cyc >> time_shift);
                  rem = cyc & ((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 & ((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.

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

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

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

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

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

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

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

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

       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.

              In addition, one of the following bits can be set:

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

              PERF_RECORD_MISC_COMM_EXEC (since Linux 3.16)
                     This is set for a PERF_RECORD_COMM record on kernels more
                     recent  than  Linux  3.16  if  a  process name change was
                     caused by an exec(2) system call.  It  is  an  alias  for
                     PERF_RECORD_MISC_MMAP_DATA since the two values would not
                     be set in the same record.

                     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).

       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

                      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   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 */
                          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 included.

                  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

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

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

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

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

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

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

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

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

                      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_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_SHIFT:

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

                          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

                             Abort  from  an  elision type transaction (Intel-

                             Abort from a generic transaction.

                             Synchronous  abort  (related  to   the   reported

                             Asynchronous  abort  (not related to the reported

                             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

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

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

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

                  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

              PERF_RECORD_AUX (since Linux 4.1)

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

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

                         offset in the AUX mmap  region  where  the  new  data

                         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

                  tid    thread  ID  of  the  thread  starting  an instruction

   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(2).   Alternately,  a
       SIGIO  signal  handler  can  be  created  and the event configured with
       fcntl(2) to generate SIGIO signals.

       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 argument.

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

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

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

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

              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 BUGS).

              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 ignored.

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

              The argument is a pointer to the desired ftrace filter.

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

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

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

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

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

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


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

                  2   allow only user-space measurements.

                  1   allow both kernel and user measurements (default).

                  0   allow access to CPU-specific data but not raw tracepoint

                  -1  no restrictions.

                  The   existence  of  the  perf_event_paranoid  file  is  the
                  official  method  for  determining  if  a  kernel   supports


                  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).


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

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

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

           /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

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

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

           /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

       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

       EFAULT Returned if  the  attr  pointer  points  at  an  invalid  memory

       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

       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

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

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

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

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

              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.

       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


       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


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

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

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

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

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

       Prior  to  Linux  2.6.35,  PERF_FORMAT_GROUP did not work with attached

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

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

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

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


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

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

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

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

       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);



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


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