perf_event_open man page on Archlinux

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PERF_EVENT_OPEN(2)	   Linux Programmer's Manual	    PERF_EVENT_OPEN(2)

NAME
       perf_event_open - set up performance monitoring

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

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

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

DESCRIPTION
       Given  a	 list of parameters, perf_event_open() returns a file descrip‐
       tor, 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 mea‐
       suring performance information.	Each file  descriptor  corresponds  to
       one  event  that	 is measured; these can be grouped together to measure
       multiple events simultaneously.

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

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

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

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

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

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

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

       pid == -1 and cpu >= 0
	      This  measures all processes/threads on the specified CPU.  This
	      requires	 CAP_SYS_ADMIN	 capability   or   a	/proc/sys/ker‐
	      nel/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
       instructions.

       The flags argument is formed by ORing together zero or more of the fol‐
       lowing 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 cre‐
	      ation time, rather than later with  fcntl(2),  avoids  potential
	      race    conditions    where    the    calling   thread   invokes
	      perf_event_open() and fcntl(2)  at  the  same  time  as  another
	      thread calls fork(2) then execve(2).

       PERF_FLAG_FD_NO_GROUP
	      This flag allows creating an event as part of an event group but
	      having no group leader.  It is unclear why this is useful.

       PERF_FLAG_FD_OUTPUT
	      This flag reroutes the output from an event to the group leader.

       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 mon‐
	      itor   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 informa‐
       tion 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 */
		     __reserved_1   : 41;

	       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 */
	       __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:

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

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

	      PERF_TYPE_TRACEPOINT
		     This indicates a tracepoint provided by the kernel trace‐
		     point infrastructure.

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

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

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

	      dynamic PMU
		     Since  Linux 2.6.39, perf_event_open() can support multi‐
		     ple 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 filesys‐
		     tem:  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  compila‐
	      tion.

	      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_ATR_SIZE_VER3 is 96 cor‐
	      responding  to  the  addition  of	 sample_regs_user   and	  sam‐
	      ple_stack_user in Linux 3.7.

       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.

	      The most significant bit (bit 63) of config  signifies  CPU-spe‐
	      cific  (raw) counter configuration data; if the most significant
	      bit is unset, the next 7 bits are an event type and the rest  of
	      the bits are the event identifier.

	      There  are  various ways to set the config field that are depen‐
	      dent 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  gen‐
	      eralized hardware CPU events.  Not all of these are available on
	      all platforms.  Set config to one of the following:

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

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

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

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

		   PERF_COUNT_HW_BRANCH_INSTRUCTIONS
			  Retired branch instructions.	Prior to Linux 2.6.34,
			  this used the wrong event on AMD processors.

		   PERF_COUNT_HW_BRANCH_MISSES
			  Mispredicted branch instructions.

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

		   PERF_COUNT_HW_STALLED_CYCLES_FRONTEND (since Linux 3.0)
			  Stalled cycles during issue.

		   PERF_COUNT_HW_STALLED_CYCLES_BACKEND (since Linux 3.0)
			  Stalled cycles during retirement.

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

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

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

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

		   PERF_COUNT_SW_PAGE_FAULTS
			  This reports the number of page faults.

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

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

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

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

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

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

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

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

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

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

		  where perf_hw_cache_id is one of:

		      PERF_COUNT_HW_CACHE_L1D
			     for measuring Level 1 Data Cache

		      PERF_COUNT_HW_CACHE_L1I
			     for measuring Level 1 Instruction Cache

		      PERF_COUNT_HW_CACHE_LL
			     for measuring Last-Level Cache

		      PERF_COUNT_HW_CACHE_DTLB
			     for measuring the Data TLB

		      PERF_COUNT_HW_CACHE_ITLB
			     for measuring the Instruction TLB

		      PERF_COUNT_HW_CACHE_BPU
			     for measuring the branch prediction unit

		      PERF_COUNT_HW_CACHE_NODE (since Linux 3.0)
			     for measuring local memory accesses

		  and perf_hw_cache_op_id is one of

		      PERF_COUNT_HW_CACHE_OP_READ
			     for read accesses

		      PERF_COUNT_HW_CACHE_OP_WRITE
			     for write accesses

		      PERF_COUNT_HW_CACHE_OP_PREFETCH
			     for prefetch accesses

		  and perf_hw_cache_op_result_id is one of

		      PERF_COUNT_HW_CACHE_RESULT_ACCESS
			     to measure accesses

		      PERF_COUNT_HW_CACHE_RESULT_MISS
			     to measure misses

	      If  type	is  PERF_TYPE_RAW, then a custom "raw" config value is
	      needed.  Most CPUs support events that are not  covered  by  the
	      "generalized"  events.   These  are  implementation defined; see
	      your CPU manual (for example the Intel Volume  3B	 documentation
	      or  the  AMD  BIOS  and  Kernel  Developer  Guide).  The libpfm4
	      library can be used to translate from the name in the  architec‐
	      tural  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" counter is one that generates an interrupt every N
	      events, where N is given by sample_period.  A  sampling  counter
	      has  sample_period  >  0.	  When	an  overflow interrupt occurs,
	      requested data is recorded in the mmap buffer.  The  sample_type
	      field controls what data is recorded on each interrupt.

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

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

	      PERF_SAMPLE_IP
		     Records instruction pointer.

	      PERF_SAMPLE_TID
		     Records the process and thread IDs.

	      PERF_SAMPLE_TIME
		     Records a timestamp.

	      PERF_SAMPLE_ADDR
		     Records an address, if applicable.

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

	      PERF_SAMPLE_CALLCHAIN
		     Records the callchain (stack backtrace).

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

	      PERF_SAMPLE_CPU
		     Records CPU number.

	      PERF_SAMPLE_PERIOD
		     Records the current sampling period.

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

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

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

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

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

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

	      PERF_SAMPLE_WEIGHT (since Linux 3.10)
		     Records a hardware provided weight value  that  expresses
		     how  costly the sampled event was.	 This allows the hard‐
		     ware 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 only available 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 differ‐
		     ent sample_type settings.	Parsing the event stream prop‐
		     erly 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).

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

	      PERF_SAMPLE_TRANSACTION (Since Linux 3.13)
		     Records reasons for  transactional	 memory	 abort	events
		     (for  example,  from  Intel TSX transactional memory sup‐
		     port).

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

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

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

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

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

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

       disabled
	      The disabled bit specifies whether the counter starts  out  dis‐
	      abled  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 ini‐
	      tialized	with  disabled	set to 1 and any child events are ini‐
	      tialized with disabled set to 0.	Despite disabled being 0,  the
	      child events will not start until the group leader is enabled.

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

	      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 coun‐
	      ters 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	subse‐
	      quently enabled or disabled.

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

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

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

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

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

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

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

       comm   The comm bit enables tracking of process command name  as	 modi‐
	      fied by the exec(2) and prctl(PR_SET_NAME) system calls.	Unfor‐
	      tunately for tools, there is no way to  distinguish  one	system
	      call versus the other.

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

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

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

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

       watermark
	      If set, have a sampling  interrupt  happen  when	we  cross  the
	      wakeup_watermark	boundary.   Otherwise  interrupts 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
		     PERF_RECORD_MISC_EXACT_IP.

       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 sam‐
	      ple_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)
	      Do not measure time spent in VM host.

       exclude_guest (since Linux 3.2)
	      Do not measure time spent in VM guest.

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

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

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

	      wakeup_events only counts PERF_RECORD_SAMPLE record  types.   To
	      receive  a  signal  for  every  incoming	PERF_RECORD  type  set
	      wakeup_watermark to 1.

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

	      HW_BREAKPOINT_EMPTY
		     No breakpoint.

	      HW_BREAKPOINT_R
		     Count when we read the memory location.

	      HW_BREAKPOINT_W
		     Count when we write the memory location.

	      HW_BREAKPOINT_RW
		     Count when we read or write the memory location.

	      HW_BREAKPOINT_X
		     Count when we execute code at the memory location.

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

       bp_addr (since Linux 2.6.33)
	      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  mem‐
	      ory 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  OFF‐
	      CORE_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,
	      HW_BREAKPOINT_LEN_2,  HW_BREAKPOINT_LEN_4,  HW_BREAKPOINT_LEN_8.
	      For an execution breakpoint, set this to sizeof(long).

       config2 (since Linux 2.6.39)

	      config2 is a further extension of the config1 field.

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

	      The first part of the value is the privilege level, which	 is  a
	      combination  of  one  of the 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.

	      PERF_SAMPLE_BRANCH_USER
		     Branch target is in user space.

	      PERF_SAMPLE_BRANCH_KERNEL
		     Branch target is in kernel space.

	      PERF_SAMPLE_BRANCH_HV
		     Branch target is in hypervisor.

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

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

	      PERF_SAMPLE_BRANCH_ANY
		     Any branch type.

	      PERF_SAMPLE_BRANCH_ANY_CALL
		     Any call branch.

	      PERF_SAMPLE_BRANCH_ANY_RETURN
		     Any return branch.

	      PERF_SAMPLE_BRANCH_IND_CALL
		     Indirect calls.

	      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.

       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-spe‐
	      cific	and	described     in     the     kernel	header
	      arch/ARCH/include/uapi/asm/perf_regs.h.

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

   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 there 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 infor‐
       mation such as where the ring-buffer head is.

       Before kernel 2.6.39, there is a bug that means	you  must  allocate  a
       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 */
	   }

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

       version
	      Version number of this structure.

       compat_version
	      The lowest version this is compatible with.

       lock   A seqlock for synchronization.

       index  A unique hardware counter identifier.

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

       time_enabled
	      Time the event was active.

       time_running
	      Time the event was running.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       time_shift, time_mult, time_offset

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

		  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);

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

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

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

       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
       perf.data 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
       first.

       size   This indicates the size of the record.

       misc   The misc field contains additional information about the sample.

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

	      PERF_RECORD_MISC_CPUMODE_UNKNOWN
		     Unknown CPU mode.

	      PERF_RECORD_MISC_KERNEL
		     Sample happened in the kernel.

	      PERF_RECORD_MISC_USER
		     Sample happened in user code.

	      PERF_RECORD_MISC_HYPERVISOR
		     Sample happened in the hypervisor.

	      PERF_RECORD_MISC_GUEST_KERNEL
		     Sample happened in the guest kernel.

	      PERF_RECORD_MISC_GUEST_USER
		     Sample happened in guest user code.

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

	      PERF_RECORD_MISC_MMAP_DATA
		     This is set when the mapping is not executable; otherwise
		     the mapping is executable.

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

	      PERF_RECORD_MISC_EXT_RESERVED
		     This  indicates  there  is	 extended data available (cur‐
		     rently not used).

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

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

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

	      PERF_RECORD_LOST
		  This record indicates when events are lost.

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

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

		  lost	 is the number of events that were lost.

	      PERF_RECORD_COMM
		  This record indicates a change in the process name.

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

	      PERF_RECORD_EXIT
		  This record indicates a process exit event.

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

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

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

	      PERF_RECORD_FORK
		  This record indicates a fork event.

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

	      PERF_RECORD_READ
		  This record indicates a read event.

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

	      PERF_RECORD_SAMPLE
		  This record indicates a sample.

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

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

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

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

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

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

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

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

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

		  period
		      If  PERF_SAMPLE_PERIOD  is enabled, a 64-bit value indi‐
		      cating 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 num‐
		      ber 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 indi‐
		      cating  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, fol‐
		      lowed by bnr  perf_branch_entry  structures  which  each
		      include the fields:

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

		      to     The branch target.

		      mispred
			     The branch target was mispredicted.

		      predicted
			     The branch target was predicted.

		      in_tx (since Linux 3.11)
			     The branch was in a transactional memory transac‐
			     tion.

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

		      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 sam‐
		      ple_regs_user bit mask.

		  size, data[size], dyn_size
		      If PERF_SAMPLE_STACK_USER is enabled,  then  record  the
		      user  stack  to  enable  backtracing.   size is the size
		      requested by the user in	stack_user_size	 or  else  the
		      maximum  record size.  data is the stack data.  dyn_size
		      is the amount of data actually dumped (can be less  than
		      size).

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

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

		      mem_op
			  Type of opcode, a bitwise combination of:

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

		      mem_lvl
			  Memory hierarchy level hit or miss, a bitwise combi‐
			  nation of:

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

		      mem_snoop
			  Snoop mode, a bitwise combination of:

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

		      mem_lock
			  Lock instruction, a bitwise combination of:

			  PERF_MEM_LOCK_NA	  Not available
			  PERF_MEM_LOCK_LOCKED	  Locked transaction

		      mem_dtlb
			  TLB access hit or miss, a bitwise combination of:

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

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

		      The field is a bitwise combination of the following val‐
		      ues:

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

		      PERF_TXN_TRANSACTION
			     Abort from a generic transaction.

		      PERF_TXN_SYNC
			     Synchronous  abort	 (related  to	the   reported
			     instruction).

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

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

		      PERF_TXN_CONFLICT
			     Abort due to memory conflicts with other threads.

		      PERF_TXN_CAPACITY_WRITE
			     Abort due to write capacity overflow.

		      PERF_TXN_CAPACITY_READ
			     Abort due to read capacity overflow.

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

   Signal overflow
       Events can be set to deliver a signal when a threshold is crossed.  The
       signal  handler	is  set	 up using the poll(2), select(2), epoll(2) and
       fcntl(2), system calls.

       To generate signals, sampling must be enabled (sample_period must  have
       a nonzero value).

       There are two ways to generate signals.

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

       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, a POLL_IN signal is sent on overflow, but once the	 value
       reaches	0,  a signal is sent of type POLL_HUP and the underlying event
       is disabled.

       Note: on newer kernels (definitely noticed with 3.2) a signal  is  pro‐
       vided for every overflow, even if wakeup_events is not set.

   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.

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

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

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

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

	      Enabling or disabling the leader of a group enables or  disables
	      the  entire  group; that is, while the group leader is disabled,
	      none of the counters in the group will count.  Enabling or  dis‐
	      abling  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 spec‐
	      ified is not the group leader (but see BUGS).

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

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

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

       PERF_EVENT_IOC_PERIOD
	      This updates the overflow period for the event.

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

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

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

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

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

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

	      The argument is a pointer to the desired ftrace filter.

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

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

   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/

	   /proc/sys/kernel/perf_event_paranoid

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

		  2   only allow user-space measurements.

		  1   allow both kernel and user measurements (default).

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

		  -1  no restrictions.

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

	   /proc/sys/kernel/perf_event_max_sample_rate

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

	   /proc/sys/kernel/perf_event_mlock_kb

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

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

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

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

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

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

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

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

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

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

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

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

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

       EACCES Returned when the requested event requires CAP_SYS_ADMIN permis‐
	      sions  (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; moni‐
	      toring all processes on a given CPU (i.e.,  specifying  the  pid
	      argument	as  -1); and not setting exclude_kernel when the para‐
	      noid 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.

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

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

       EMFILE Each  opened  event uses one file descriptor.  If a large number
	      of events are opened the per-user file descriptor	 limit	(often
	      1024) will be hit and no more events can be created.

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

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

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

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

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

       EPERM  Returned on many (but not all) architectures when an unsupported
	      exclude_hv, exclude_idle, exclude_user, or  exclude_kernel  set‐
	      ting 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 break‐
	      point on a kernel address, and (since Linux 3.13) setting a ker‐
	      nel function-trace tracepoint.

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

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

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

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

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

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

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

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

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

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

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

       In older Linux 2.6 versions, refreshing an event group leader refreshed
       all siblings, and refreshing with a parameter  of  0  enabled  infinite
       refresh.	 This behavior is unsupported and should not be relied on.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	   close(fd);
       }

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

COLOPHON
       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2014-04-17		    PERF_EVENT_OPEN(2)
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