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.
When pid is greater than zero, permission to perform this system call
is governed by a ptrace access mode PTRACE_MODE_READ_REALCREDS check;
see ptrace(2).
The group_fd argument allows event groups to be created. An event
group has one event which is the group leader. The leader is created
first, with group_fd = -1. The rest of the group members are created
with subsequent perf_event_open() calls with group_fd being set to the
file descriptor of the group leader. (A single event on its own is
created with group_fd = -1 and is considered to be a group with only 1
member.) An event group is scheduled onto the CPU as a unit: it will
be put onto the CPU only if all of the events in the group can be put
onto the CPU. This means that the values of the member events can be
meaningfully compared--added, divided (to get ratios), and so on--with
each other, since they have counted events for the same set of executed
instructions.
The flags argument is formed by ORing together zero or more of the 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 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 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 */
mmap2 : 1, /* include mmap with inode data */
comm_exec : 1, /* flag comm events that are
due to exec */
use_clockid : 1, /* use clockid for time fields */
context_switch : 1, /* context switch data */
__reserved_1 : 37;
union {
__u32 wakeup_events; /* wakeup every n events */
__u32 wakeup_watermark; /* bytes before wakeup */
};
__u32 bp_type; /* breakpoint type */
union {
__u64 bp_addr; /* breakpoint address */
__u64 config1; /* extension of config */
};
union {
__u64 bp_len; /* breakpoint length */
__u64 config2; /* extension of config1 */
};
__u64 branch_sample_type; /* enum perf_branch_sample_type */
__u64 sample_regs_user; /* user regs to dump on samples */
__u32 sample_stack_user; /* size of stack to dump on
samples */
__s32 clockid; /* clock to use for time fields */
__u64 sample_regs_intr; /* regs to dump on samples */
__u32 aux_watermark; /* aux bytes before wakeup */
__u16 sample_max_stack; /* max frames in callchain */
__u16 __reserved_2; /* align to u64 */
};
The fields of the perf_event_attr structure are described in more
detail below:
type This field specifies the overall event type. It has one of the
following values:
PERF_TYPE_HARDWARE
This indicates one of the "generalized" hardware events
provided by the kernel. See the config field definition
for more details.
PERF_TYPE_SOFTWARE
This indicates one of the software-defined events 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 2.6.38, 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_ATTR_SIZE_VER3 is 96 cor-
responding to the addition of sample_regs_user and sam-
ple_stack_user in Linux 3.7. PERF_ATTR_SIZE_VER4 is 104 corre-
sponding to the addition of sample_regs_intr in Linux 3.19.
PERF_ATTR_SIZE_VER5 is 112 corresponding to the addition of
aux_watermark in Linux 4.1.
config This specifies which event you want, in conjunction with the
type field. The config1 and config2 fields are also taken into
account in cases where 64 bits is not enough to fully specify
the event. The encoding of these fields are event dependent.
There are various ways to set the config field that are 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 2.6.35,
this used the wrong event on AMD processors.
PERF_COUNT_HW_BRANCH_MISSES
Mispredicted branch instructions.
PERF_COUNT_HW_BUS_CYCLES
Bus cycles, which can be different from total
cycles.
PERF_COUNT_HW_STALLED_CYCLES_FRONTEND (since Linux 3.0)
Stalled cycles during issue.
PERF_COUNT_HW_STALLED_CYCLES_BACKEND (since Linux 3.0)
Stalled cycles during retirement.
PERF_COUNT_HW_REF_CPU_CYCLES (since Linux 3.3)
Total cycles; not affected by CPU frequency scaling.
If type is PERF_TYPE_SOFTWARE, we are measuring software events
provided by the kernel. Set config to one of the following:
PERF_COUNT_SW_CPU_CLOCK
This reports the CPU clock, a high-resolution per-
CPU timer.
PERF_COUNT_SW_TASK_CLOCK
This reports a clock count specific to the task that
is running.
PERF_COUNT_SW_PAGE_FAULTS
This reports the number of page faults.
PERF_COUNT_SW_CONTEXT_SWITCHES
This counts context switches. Until Linux 2.6.34,
these were all reported as user-space events, after
that they are reported as happening in the kernel.
PERF_COUNT_SW_CPU_MIGRATIONS
This reports the number of times the process has
migrated to a new CPU.
PERF_COUNT_SW_PAGE_FAULTS_MIN
This counts the number of minor page faults. These
did not require disk I/O to handle.
PERF_COUNT_SW_PAGE_FAULTS_MAJ
This counts the number of major page faults. These
required disk I/O to handle.
PERF_COUNT_SW_ALIGNMENT_FAULTS (since Linux 2.6.33)
This counts the number of alignment faults. These
happen when unaligned memory accesses happen; the
kernel can handle these but it reduces performance.
This happens only on some architectures (never on
x86).
PERF_COUNT_SW_EMULATION_FAULTS (since Linux 2.6.33)
This counts the number of emulation faults. The
kernel sometimes traps on unimplemented instructions
and emulates them for user space. This can 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 3.1)
for measuring local memory accesses
and perf_hw_cache_op_id is one of:
PERF_COUNT_HW_CACHE_OP_READ
for read accesses
PERF_COUNT_HW_CACHE_OP_WRITE
for write accesses
PERF_COUNT_HW_CACHE_OP_PREFETCH
for prefetch accesses
and perf_hw_cache_op_result_id is one of:
PERF_COUNT_HW_CACHE_RESULT_ACCESS
to measure accesses
PERF_COUNT_HW_CACHE_RESULT_MISS
to measure misses
If type is PERF_TYPE_RAW, then a custom "raw" config value is
needed. Most CPUs support events that are not covered by the
"generalized" events. These are implementation defined; see
your CPU manual (for example the Intel Volume 3B documentation
or the AMD BIOS and Kernel Developer Guide). The libpfm4
library can be used to translate from the name in the 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" event is one that generates an overflow notifica-
tion every N events, where N is given by sample_period. A sam-
pling event has sample_period > 0. When an overflow occurs,
requested data is recorded in the mmap buffer. The sample_type
field controls what data is recorded on each overflow.
sample_freq can be used if you wish to use frequency rather than
period. In this case, you set the freq flag. The kernel will
adjust the sampling period to try and achieve the desired rate.
The rate of adjustment is a timer tick.
sample_type
The various bits in this field specify which values to include
in the sample. They will be recorded in a ring-buffer, which is
available to user space using mmap(2). The order in which the
values are saved in the sample are documented in the MMAP Layout
subsection below; it is not the enum perf_event_sample_format
order.
PERF_SAMPLE_IP
Records instruction pointer.
PERF_SAMPLE_TID
Records the process and thread IDs.
PERF_SAMPLE_TIME
Records a timestamp.
PERF_SAMPLE_ADDR
Records an address, if applicable.
PERF_SAMPLE_READ
Record counter values for all events in a group, not just
the group leader.
PERF_SAMPLE_CALLCHAIN
Records the callchain (stack backtrace).
PERF_SAMPLE_ID
Records a unique ID for the opened event's group leader.
PERF_SAMPLE_CPU
Records CPU number.
PERF_SAMPLE_PERIOD
Records the current sampling period.
PERF_SAMPLE_STREAM_ID
Records a unique ID for the opened event. Unlike
PERF_SAMPLE_ID the actual ID is returned, not the group
leader. This ID is the same as the one returned by
PERF_FORMAT_ID.
PERF_SAMPLE_RAW
Records additional data, if applicable. Usually returned
by tracepoint events.
PERF_SAMPLE_BRANCH_STACK (since Linux 3.4)
This provides a record of recent branches, as provided by
CPU branch sampling hardware (such as Intel Last Branch
Record). Not all hardware supports this feature.
See the branch_sample_type field for how to filter which
branches are reported.
PERF_SAMPLE_REGS_USER (since Linux 3.7)
Records the current user-level CPU register state (the
values in the process before the kernel was called).
PERF_SAMPLE_STACK_USER (since Linux 3.7)
Records the user level stack, allowing stack unwinding.
PERF_SAMPLE_WEIGHT (since Linux 3.10)
Records a hardware provided weight value that expresses
how costly the sampled event was. This allows the 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 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 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).
The PERF_SAMPLE_IDENTIFIER setting makes the event stream
always parsable by putting SAMPLE_ID in a fixed location,
even though it means having duplicate SAMPLE_ID values in
records.
PERF_SAMPLE_TRANSACTION (since Linux 3.13)
Records reasons for transactional memory abort events
(for example, from Intel TSX transactional memory 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).
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_SAM-
PLE_REGS_USER the register values will return kernel reg-
ister state if the overflow happened while kernel code is
running. If the CPU supports hardware sampling of regis-
ter state (i.e., PEBS on Intel x86) and precise_ip is set
higher than zero then the register values returned are
those captured by hardware at the time of the sampled
instruction's retirement.
read_format
This field specifies the format of the data returned by read(2)
on a perf_event_open() file descriptor.
PERF_FORMAT_TOTAL_TIME_ENABLED
Adds the 64-bit time_enabled field. This can be used to
calculate estimated totals if the PMU is overcommitted
and multiplexing is happening.
PERF_FORMAT_TOTAL_TIME_RUNNING
Adds the 64-bit time_running field. This can be used to
calculate estimated totals if the PMU is overcommitted
and multiplexing is happening.
PERF_FORMAT_ID
Adds a 64-bit unique value that corresponds to the event
group.
PERF_FORMAT_GROUP
Allows all counter values in an event group to be read
with one read.
disabled
The disabled bit specifies whether the counter starts out 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_format val-
ues, 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 as well
as writing to /proc/self/comm. If the comm_exec flag is also
successfully set (possible since Linux 3.16), then the misc flag
PERF_RECORD_MISC_COMM_EXEC can be used to differentiate the
exec(2) case from the others.
freq If this bit is set, then sample_frequency not sample_period is
used when setting up the sampling interval.
inherit_stat
This bit enables saving of event counts on context switch for
inherited tasks. This is meaningful only if the inherit field
is set.
enable_on_exec
If this bit is set, a counter is automatically enabled after a
call to exec(2).
task If this bit is set, then fork/exit notifications are included in
the ring buffer.
watermark
If set, have an overflow notification happen when we cross the
wakeup_watermark boundary. Otherwise, overflow notifications
happen after wakeup_events samples.
precise_ip (since Linux 2.6.35)
This controls the amount of skid. Skid is how many instructions
execute between an event of interest happening and the kernel
being able to stop and record the event. Smaller skid is better
and allows more accurate reporting of which events correspond to
which instructions, but hardware is often limited with how small
this can be.
The possible values of this field are the following:
0 SAMPLE_IP can have arbitrary skid.
1 SAMPLE_IP must have constant skid.
2 SAMPLE_IP requested to have 0 skid.
3 SAMPLE_IP must have 0 skid. See also the description of
PERF_RECORD_MISC_EXACT_IP.
mmap_data (since Linux 2.6.36)
This is the counterpart of the mmap field. This enables genera-
tion 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 */
};
,in
exclude_host (since Linux 3.2)
When conducting measurements that include processes running VM
instances (i.e., have executed a KVM_RUN ioctl(2)), only measure
events happening inside a guest instance. This is only meaning-
ful 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 mea-
sure 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 functional-
ity 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 ker-
nel 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 renam-
ing.
use_clockid (since Linux 4.1)
This allows selecting which internal Linux clock to use when
generating timestamps via the clockid field. This can make it
easier to correlate perf sample times with timestamps generated
by other tools.
context_switch (since Linux 4.3)
This enables the generation of PERF_RECORD_SWITCH records when a
context switch occurs. It also enables the generation of
PERF_RECORD_SWITCH_CPU_WIDE records when sampling in CPU-wide
mode. This functionality is in addition to existing tracepoint
and software events for measuring context switches. The advan-
tage of this method is that it will give full information even
with strict perf_event_paranoid settings.
wakeup_events, wakeup_watermark
This union sets how many samples (wakeup_events) or bytes
(wakeup_watermark) happen before an overflow notification hap-
pens. Which one is used is selected by the watermark bit flag.
wakeup_events counts only PERF_RECORD_SAMPLE record types. To
receive overflow notification for all PERF_RECORD types choose
watermark and set wakeup_watermark to 1.
Prior to Linux 3.0, setting wakeup_events to 0 resulted in no
overflow notifications; more recent kernels treat 0 the same as
1.
bp_type (since Linux 2.6.33)
This chooses the breakpoint type. It is one of:
HW_BREAKPOINT_EMPTY
No breakpoint.
HW_BREAKPOINT_R
Count when we read the memory location.
HW_BREAKPOINT_W
Count when we write the memory location.
HW_BREAKPOINT_RW
Count when we read or write the memory location.
HW_BREAKPOINT_X
Count when we execute code at the memory location.
The values can be combined via a bitwise or, but the combination
of HW_BREAKPOINT_R or HW_BREAKPOINT_W with HW_BREAKPOINT_X is
not allowed.
bp_addr (since Linux 2.6.33)
This is the address of the breakpoint. For execution break-
points, this is the memory address of the instruction of inter-
est; 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 OFF-
CORE_EVENTS on Nehalem/Westmere/SandyBridge use this field on
Linux 3.3 and later kernels.
bp_len (since Linux 2.6.33)
bp_len is the length of the breakpoint being measured if type is
PERF_TYPE_BREAKPOINT. Options are HW_BREAKPOINT_LEN_1,
HW_BREAKPOINT_LEN_2, HW_BREAKPOINT_LEN_4, and HW_BREAK-
POINT_LEN_8. For an execution breakpoint, set this to
sizeof(long).
config2 (since Linux 2.6.39)
config2 is a further extension of the config1 field.
branch_sample_type (since Linux 3.4)
If PERF_SAMPLE_BRANCH_STACK is enabled, then this specifies what
branches to include in the branch record.
The first part of the value is the privilege level, which is a
combination of one of the values listed below. If the user does
not set privilege level explicitly, the kernel will use the
event's privilege level. Event and branch privilege levels do
not have to match.
PERF_SAMPLE_BRANCH_USER
Branch target is in user space.
PERF_SAMPLE_BRANCH_KERNEL
Branch target is in kernel space.
PERF_SAMPLE_BRANCH_HV
Branch target is in hypervisor.
PERF_SAMPLE_BRANCH_PLM_ALL
A convenience value that is the three preceding values
ORed together.
In addition to the privilege value, at least one or more of the
following bits must be set.
PERF_SAMPLE_BRANCH_ANY
Any branch type.
PERF_SAMPLE_BRANCH_ANY_CALL
Any call branch (includes direct calls, indirect calls,
and far jumps).
PERF_SAMPLE_BRANCH_IND_CALL
Indirect calls.
PERF_SAMPLE_BRANCH_CALL (since Linux 4.4)
Direct calls.
PERF_SAMPLE_BRANCH_ANY_RETURN
Any return branch.
PERF_SAMPLE_BRANCH_IND_JUMP (since Linux 4.2)
Indirect jumps.
PERF_SAMPLE_BRANCH_COND (since Linux 3.16)
Conditional branches.
PERF_SAMPLE_BRANCH_ABORT_TX (since Linux 3.11)
Transactional memory aborts.
PERF_SAMPLE_BRANCH_IN_TX (since Linux 3.11)
Branch in transactional memory transaction.
PERF_SAMPLE_BRANCH_NO_TX (since Linux 3.11)
Branch not in transactional memory transaction.
PERF_SAMPLE_BRANCH_CALL_STACK (since Linux 4.1) Branch is
part of a hardware-generated call stack. This requires
hardware support, currently only found on Intel x86
Haswell or newer.
sample_regs_user (since Linux 3.7)
This bit mask defines the set of user CPU registers to dump on
samples. The layout of the register mask is architecture-spe-
cific and is described in the kernel header file
arch/ARCH/include/uapi/asm/perf_regs.h.
sample_stack_user (since Linux 3.7)
This defines the size of the user stack to dump if PERF_SAM-
PLE_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_MONO-
TONIC_RAW, CLOCK_REALTIME, CLOCK_BOOTTIME, and CLOCK_TAI cur-
rently supported.
aux_watermark (since Linux 4.1)
This specifies how much data is required to trigger a
PERF_RECORD_AUX sample.
sample_max_stack (since Linux 4.8)
When sample_type includes PERF_SAMPLE_CALLCHAIN, this field
specifies how many stack frames to report when generating the
callchain.
Reading results
Once a perf_event_open() file descriptor has been opened, the values of
the events can be read from the file descriptor. The values that are
there are specified by the read_format field in the attr structure at
open time.
If you attempt to read into a buffer that is not big enough to hold the
data, the error ENOSPC results.
Here is the layout of the data returned by a read:
* If PERF_FORMAT_GROUP was specified to allow reading all events in a
group at once:
struct read_format {
u64 nr; /* The number of events */
u64 time_enabled; /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
u64 time_running; /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
struct {
u64 value; /* The value of the event */
u64 id; /* if PERF_FORMAT_ID */
} values[nr];
};
* If PERF_FORMAT_GROUP was not specified:
struct read_format {
u64 value; /* The value of the event */
u64 time_enabled; /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
u64 time_running; /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
u64 id; /* if PERF_FORMAT_ID */
};
The values read are as follows:
nr The number of events in this file descriptor. Available only if
PERF_FORMAT_GROUP was specified.
time_enabled, time_running
Total time the event was enabled and running. Normally these
values are the same. If more events are started, then available
counter slots on the PMU, then multiplexing happens and events
run only part of the time. In that case, the time_enabled and
time running values can be used to scale an estimated value for
the count.
value An unsigned 64-bit value containing the counter result.
id A globally unique value for this particular event; only present
if PERF_FORMAT_ID was specified in read_format.
MMAP layout
When using perf_event_open() in sampled mode, asynchronous events (like
counter overflow or PROT_EXEC mmap tracking) are logged into a ring-
buffer. This ring-buffer is created and accessed through mmap(2).
The mmap size should be 1+2^n pages, where the first page is a metadata
page (struct perf_event_mmap_page) that contains various bits of infor-
mation such as where the ring-buffer head is.
Before kernel 2.6.39, there is a bug that means you must allocate an
mmap ring buffer when sampling even if you do not plan to access it.
The structure of the first metadata mmap page is as follows:
struct perf_event_mmap_page {
__u32 version; /* version number of this structure */
__u32 compat_version; /* lowest version this is compat with */
__u32 lock; /* seqlock for synchronization */
__u32 index; /* hardware counter identifier */
__s64 offset; /* add to hardware counter value */
__u64 time_enabled; /* time event active */
__u64 time_running; /* time event on CPU */
union {
__u64 capabilities;
struct {
__u64 cap_usr_time / cap_usr_rdpmc / cap_bit0 : 1,
cap_bit0_is_deprecated : 1,
cap_user_rdpmc : 1,
cap_user_time : 1,
cap_user_time_zero : 1,
};
};
__u16 pmc_width;
__u16 time_shift;
__u32 time_mult;
__u64 time_offset;
__u64 __reserved[120]; /* Pad to 1 k */
__u64 data_head; /* head in the data section */
__u64 data_tail; /* user-space written tail */
__u64 data_offset; /* where the buffer starts */
__u64 data_size; /* data buffer size */
__u64 aux_head;
__u64 aux_tail;
__u64 aux_offset;
__u64 aux_size;
}
The following list describes the fields in the perf_event_mmap_page
structure in more detail:
version
Version number of this structure.
compat_version
The lowest version this is compatible with.
lock A seqlock for synchronization.
index A unique hardware counter identifier.
offset When using rdpmc for reads this offset value must be added to
the one returned by rdpmc to get the current total event count.
time_enabled
Time the event was active.
time_running
Time the event was running.
cap_usr_time / cap_usr_rdpmc / cap_bit0 (since Linux 3.4)
There was a bug in the definition of cap_usr_time and
cap_usr_rdpmc from Linux 3.4 until Linux 3.11. Both bits were
defined to point to the same location, so it was impossible to
know if cap_usr_time or cap_usr_rdpmc were actually set.
Starting with Linux 3.12, these are renamed to cap_bit0 and you
should use the cap_user_time and cap_user_rdpmc fields instead.
cap_bit0_is_deprecated (since Linux 3.12)
If set, this bit indicates that the kernel supports the properly
separated cap_user_time and cap_user_rdpmc bits.
If not-set, it indicates an older kernel where cap_usr_time and
cap_usr_rdpmc map to the same bit and thus both features should
be used with caution.
cap_user_rdpmc (since Linux 3.12)
If the hardware supports user-space read of performance counters
without syscall (this is the "rdpmc" instruction on x86), then
the following code can be used to do a read:
u32 seq, time_mult, time_shift, idx, width;
u64 count, enabled, running;
u64 cyc, time_offset;
do {
seq = pc->lock;
barrier();
enabled = pc->time_enabled;
running = pc->time_running;
if (pc->cap_usr_time && enabled != running) {
cyc = rdtsc();
time_offset = pc->time_offset;
time_mult = pc->time_mult;
time_shift = pc->time_shift;
}
idx = pc->index;
count = pc->offset;
if (pc->cap_usr_rdpmc && idx) {
width = pc->pmc_width;
count += rdpmc(idx - 1);
}
barrier();
} while (pc->lock != seq);
cap_user_time (since Linux 3.12)
This bit indicates the hardware has a constant, nonstop 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 & (((u64)1 << time_shift) - 1);
delta = time_offset + quot * time_mult +
((rem * time_mult) >> time_shift);
Where time_offset, time_mult, time_shift, and cyc are read in
the seqcount loop described above. This delta can then be added
to enabled and possible running (if idx), improving the scaling:
enabled += delta;
if (idx)
running += delta;
quot = count / running;
rem = count % running;
count = quot * enabled + (rem * enabled) / running;
time_zero (since Linux 3.12)
If cap_usr_time_zero is set, then the hardware clock (the TSC
timestamp counter on x86) can be calculated from the time_zero,
time_mult, and time_shift values:
time = timestamp - time_zero;
quot = time / time_mult;
rem = time % time_mult;
cyc = (quot << time_shift) + (rem << time_shift) / time_mult;
And vice versa:
quot = cyc >> time_shift;
rem = cyc & (((u64)1 << time_shift) - 1);
timestamp = time_zero + quot * time_mult +
((rem * time_mult) >> time_shift);
data_head
This points to the head of the data section. The value 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.
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 buf-
fer.
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 buf-
fer). 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 buf-
fer mode where old data will be overwritten by new. In over-
write mode, it might not be possible to infer where the new data
began, and it is the consumer's job to disable measurement while
reading to avoid possible data races.
The aux_head and aux_tail ring buffer pointers have the same
behavior and ordering rules as the previous described data_head
and data_tail.
The following 2^n ring-buffer pages have the layout described below.
If perf_event_attr.sample_id_all is set, then all event types will have
the sample_type selected fields related to where/when (identity) an
event took place (TID, TIME, ID, CPU, STREAM_ID) described in
PERF_RECORD_SAMPLE below, it will be stashed just after the
perf_event_header and the fields already present for the existing
fields, that is, at the end of the payload. This allows a newer
perf.data file to be supported by older perf tools, with the new
optional fields being ignored.
The mmap values start with a header:
struct perf_event_header {
__u32 type;
__u16 misc;
__u16 size;
};
Below, we describe the perf_event_header fields in more detail. For
ease of reading, the fields with shorter descriptions are presented
first.
size This indicates the size of the record.
misc The misc field contains additional information about the sample.
The CPU mode can be determined from this value by masking with
PERF_RECORD_MISC_CPUMODE_MASK and looking for one of the 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 (since Linux 2.6.35)
Sample happened in the guest kernel.
PERF_RECORD_MISC_GUEST_USER (since Linux 2.6.35)
Sample happened in guest user code.
Since the following three statuses are generated by different
record types, they alias to the same bit:
PERF_RECORD_MISC_MMAP_DATA (since Linux 3.10)
This is set when the mapping is not executable; otherwise
the mapping is executable.
PERF_RECORD_MISC_COMM_EXEC (since Linux 3.16)
This is set for a PERF_RECORD_COMM record on kernels more
recent than Linux 3.16 if a process name change was
caused by an exec(2) system call.
PERF_RECORD_MISC_SWITCH_OUT (since Linux 4.3)
When a PERF_RECORD_SWITCH or PERF_RECORD_SWITCH_CPU_WIDE
record is generated, this bit indicates that the context
switch is away from the current process (instead of into
the current process).
In addition, the following bits can be set:
PERF_RECORD_MISC_EXACT_IP
This indicates that the content of PERF_SAMPLE_IP points
to the actual instruction that triggered the event. See
also perf_event_attr.precise_ip.
PERF_RECORD_MISC_EXT_RESERVED (since Linux 2.6.35)
This indicates there is extended data available (cur-
rently not used).
PERF_RECORD_MISC_PROC_MAP_PARSE_TIMEOUT
This bit is not set by the kernel. It is reserved for
the user-space perf utility to indicate that
/proc/i[pid]/maps parsing was taking too long and was
stopped, and thus the mmap records may be truncated.
type The type value is one of the below. The values in the 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[];
};
pid is the process ID.
tid is the thread ID.
addr is the address of the allocated memory. len is the
length of the allocated memory. pgoff is the page
offset of the allocated memory. filename is a string
describing the backing of the allocated memory.
PERF_RECORD_LOST
This record indicates when events are lost.
struct {
struct perf_event_header header;
u64 id;
u64 lost;
struct sample_id sample_id;
};
id is the unique event ID for the samples that were
lost.
lost is the number of events that were lost.
PERF_RECORD_COMM
This record indicates a change in the process name.
struct {
struct perf_event_header header;
u32 pid;
u32 tid;
char comm[];
struct sample_id sample_id;
};
pid is the process ID.
tid is the thread ID.
comm is a string containing the new name of the process.
PERF_RECORD_EXIT
This record indicates a process exit event.
struct {
struct perf_event_header header;
u32 pid, ppid;
u32 tid, ptid;
u64 time;
struct sample_id sample_id;
};
PERF_RECORD_THROTTLE, PERF_RECORD_UNTHROTTLE
This record indicates a throttle/unthrottle event.
struct {
struct perf_event_header header;
u64 time;
u64 id;
u64 stream_id;
struct sample_id sample_id;
};
PERF_RECORD_FORK
This record indicates a fork event.
struct {
struct perf_event_header header;
u32 pid, ppid;
u32 tid, ptid;
u64 time;
struct sample_id sample_id;
};
PERF_RECORD_READ
This record indicates a read event.
struct {
struct perf_event_header header;
u32 pid, tid;
struct read_format values;
struct sample_id sample_id;
};
PERF_RECORD_SAMPLE
This record indicates a sample.
struct {
struct perf_event_header header;
u64 sample_id; /* if PERF_SAMPLE_IDENTIFIER */
u64 ip; /* if PERF_SAMPLE_IP */
u32 pid, tid; /* if PERF_SAMPLE_TID */
u64 time; /* if PERF_SAMPLE_TIME */
u64 addr; /* if PERF_SAMPLE_ADDR */
u64 id; /* if PERF_SAMPLE_ID */
u64 stream_id; /* if PERF_SAMPLE_STREAM_ID */
u32 cpu, res; /* if PERF_SAMPLE_CPU */
u64 period; /* if PERF_SAMPLE_PERIOD */
struct read_format v;
/* if PERF_SAMPLE_READ */
u64 nr; /* if PERF_SAMPLE_CALLCHAIN */
u64 ips[nr]; /* if PERF_SAMPLE_CALLCHAIN */
u32 size; /* if PERF_SAMPLE_RAW */
char data[size]; /* if PERF_SAMPLE_RAW */
u64 bnr; /* if PERF_SAMPLE_BRANCH_STACK */
struct perf_branch_entry lbr[bnr];
/* if PERF_SAMPLE_BRANCH_STACK */
u64 abi; /* if PERF_SAMPLE_REGS_USER */
u64 regs[weight(mask)];
/* if PERF_SAMPLE_REGS_USER */
u64 size; /* if PERF_SAMPLE_STACK_USER */
char data[size]; /* if PERF_SAMPLE_STACK_USER */
u64 dyn_size; /* if PERF_SAMPLE_STACK_USER &&
size != 0 */
u64 weight; /* if PERF_SAMPLE_WEIGHT */
u64 data_src; /* if PERF_SAMPLE_DATA_SRC */
u64 transaction; /* if PERF_SAMPLE_TRANSACTION */
u64 abi; /* if PERF_SAMPLE_REGS_INTR */
u64 regs[weight(mask)];
/* if PERF_SAMPLE_REGS_INTR */
};
sample_id
If PERF_SAMPLE_IDENTIFIER is enabled, a 64-bit unique ID
is included. This is a duplication of the PERF_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.
cycles (since Linux 4.3)
This reports the number of cycles elapsed since
the previous branch stack update.
The entries are from most to least recent, so the first
entry has the most recent branch.
Support for mispred, predicted, and cycles is optional;
if not supported, those values will be 0.
The type of branches recorded is specified by the
branch_sample_type field.
abi, regs[weight(mask)]
If PERF_SAMPLE_REGS_USER is enabled, then the user CPU
registers are recorded.
The abi field is one of PERF_SAMPLE_REGS_ABI_NONE,
PERF_SAMPLE_REGS_ABI_32 or PERF_SAMPLE_REGS_ABI_64.
The regs field is an array of the CPU registers that
were specified by the sample_regs_user attr field. The
number of values is the number of bits set in the sam-
ple_regs_user bit mask.
size, data[size], dyn_size
If PERF_SAMPLE_STACK_USER is enabled, then the user
stack is recorded. This can be used to generate stack
backtraces. size is the size requested by the user in
sample_stack_user or else the maximum record size. data
is the stack data (a raw dump of the memory pointed to
by the stack pointer at the time of sampling). dyn_size
is the amount of data actually dumped (can be less than
size). Note that dyn_size is omitted if size is 0.
weight
If PERF_SAMPLE_WEIGHT is enabled, then a 64-bit value
provided by the hardware is recorded that indicates how
costly the event was. This allows expensive events to
stand out more clearly in profiles.
data_src
If PERF_SAMPLE_DATA_SRC is enabled, then a 64-bit value
is recorded that is made up of the following fields:
mem_op
Type of opcode, a bitwise combination of:
PERF_MEM_OP_NA Not available
PERF_MEM_OP_LOAD Load instruction
PERF_MEM_OP_STORE Store instruction
PERF_MEM_OP_PFETCH Prefetch
PERF_MEM_OP_EXEC Executable code
mem_lvl
Memory hierarchy level hit or miss, a bitwise combi-
nation of the following, shifted left by
PERF_MEM_LVL_SHIFT:
PERF_MEM_LVL_NA Not available
PERF_MEM_LVL_HIT Hit
PERF_MEM_LVL_MISS Miss
PERF_MEM_LVL_L1 Level 1 cache
PERF_MEM_LVL_LFB Line fill buffer
PERF_MEM_LVL_L2 Level 2 cache
PERF_MEM_LVL_L3 Level 3 cache
PERF_MEM_LVL_LOC_RAM Local DRAM
PERF_MEM_LVL_REM_RAM1 Remote DRAM 1 hop
PERF_MEM_LVL_REM_RAM2 Remote DRAM 2 hops
PERF_MEM_LVL_REM_CCE1 Remote cache 1 hop
PERF_MEM_LVL_REM_CCE2 Remote cache 2 hops
PERF_MEM_LVL_IO I/O memory
PERF_MEM_LVL_UNC Uncached memory
mem_snoop
Snoop mode, a bitwise combination of the following,
shifted left by PERF_MEM_SNOOP_SHIFT:
PERF_MEM_SNOOP_NA Not available
PERF_MEM_SNOOP_NONE No snoop
PERF_MEM_SNOOP_HIT Snoop hit
PERF_MEM_SNOOP_MISS Snoop miss
PERF_MEM_SNOOP_HITM Snoop hit modified
mem_lock
Lock instruction, a bitwise combination of the fol-
lowing, shifted left by PERF_MEM_LOCK_SHIFT:
PERF_MEM_LOCK_NA Not available
PERF_MEM_LOCK_LOCKED Locked transaction
mem_dtlb
TLB access hit or miss, a bitwise combination of the
following, shifted left by PERF_MEM_TLB_SHIFT:
PERF_MEM_TLB_NA Not available
PERF_MEM_TLB_HIT Hit
PERF_MEM_TLB_MISS Miss
PERF_MEM_TLB_L1 Level 1 TLB
PERF_MEM_TLB_L2 Level 2 TLB
PERF_MEM_TLB_WK Hardware walker
PERF_MEM_TLB_OS OS fault handler
transaction
If the PERF_SAMPLE_TRANSACTION flag is set, then a
64-bit field is recorded describing the sources of any
transactional memory aborts.
The field is a bitwise combination of the following 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 the value
PERF_TXN_ABORT_MASK.
abi, regs[weight(mask)]
If PERF_SAMPLE_REGS_INTR is enabled, then the user CPU
registers are recorded.
The abi field is one of PERF_SAMPLE_REGS_ABI_NONE,
PERF_SAMPLE_REGS_ABI_32, or PERF_SAMPLE_REGS_ABI_64.
The regs field is an array of the CPU registers that
were specified by the sample_regs_intr attr field. The
number of values is the number of bits set in the sam-
ple_regs_intr bit mask.
PERF_RECORD_MMAP2
This record includes extended information on mmap(2) calls
returning executable mappings. The format is similar to
that of the PERF_RECORD_MMAP record, but includes extra val-
ues that allow uniquely identifying shared mappings.
struct {
struct perf_event_header header;
u32 pid;
u32 tid;
u64 addr;
u64 len;
u64 pgoff;
u32 maj;
u32 min;
u64 ino;
u64 ino_generation;
u32 prot;
u32 flags;
char filename[];
struct sample_id sample_id;
};
pid is the process ID.
tid is the thread ID.
addr is the address of the allocated memory.
len is the length of the allocated memory.
pgoff is the page offset of the allocated memory.
maj is the major ID of the underlying device.
min is the minor ID of the underlying device.
ino is the inode number.
ino_generation
is the inode generation.
prot is the protection information.
flags is the flags information.
filename
is a string describing the backing of the allocated
memory.
PERF_RECORD_AUX (since Linux 4.1)
This record reports that new data is available in the sepa-
rate AUX buffer region.
struct {
struct perf_event_header header;
u64 aux_offset;
u64 aux_size;
u64 flags;
struct sample_id sample_id;
};
aux_offset
offset in the AUX mmap region where the new data
begins.
aux_size
size of the data made available.
flags describes the AUX update.
PERF_AUX_FLAG_TRUNCATED
if set, then the data returned was truncated
to fit the available buffer size.
PERF_AUX_FLAG_OVERWRITE
if set, then the data returned has overwritten
previous data.
PERF_RECORD_ITRACE_START (since Linux 4.1)
This record indicates which process has initiated an
instruction trace event, allowing tools to properly corre-
late the instruction addresses in the AUX buffer with the
proper executable.
struct {
struct perf_event_header header;
u32 pid;
u32 tid;
};
pid process ID of the thread starting an instruction
trace.
tid thread ID of the thread starting an instruction
trace.
PERF_RECORD_LOST_SAMPLES (since Linux 4.2)
When using hardware sampling (such as Intel PEBS) this
record indicates some number of samples that may have been
lost.
struct {
struct perf_event_header header;
u64 lost;
struct sample_id sample_id;
};
lost the number of potentially lost samples.
PERF_RECORD_SWITCH (since Linux 4.3)
This record indicates a context switch has happened. The
PERF_RECORD_MISC_SWITCH_OUT bit in the misc field indicates
whether it was a context switch into or away from the cur-
rent process.
struct {
struct perf_event_header header;
struct sample_id sample_id;
};
PERF_RECORD_SWITCH_CPU_WIDE (since Linux 4.3)
As with PERF_RECORD_SWITCH this record indicates a context
switch has happened, but it only occurs when sampling in
CPU-wide mode and provides additional information on the
process being switched to/from. The
PERF_RECORD_MISC_SWITCH_OUT bit in the misc field indicates
whether it was a context switch into or away from the cur-
rent process.
struct {
struct perf_event_header header;
u32 next_prev_pid;
u32 next_prev_tid;
struct sample_id sample_id;
};
next_prev_pid
The process ID of the previous (if switching in) or
next (if switching out) process on the CPU.
next_prev_tid
The thread ID of the previous (if switching in) or
next (if switching out) thread on the CPU.
Overflow handling
Events can be set to notify when a threshold is crossed, indicating an
overflow. Overflow conditions can be captured by monitoring the event
file descriptor with poll(2), select(2), or epoll(7). Alternatively,
the overflow events can be captured via sa signal handler, by enabling
I/O signaling on the file descriptor; see the discussion of the
F_SETOWN and F_SETSIG operations in fcntl(2).
Overflows are generated only by sampling events (sample_period must
have a nonzero value).
There are two ways to generate overflow notifications.
The first is to set a wakeup_events or wakeup_watermark value that will
trigger if a certain number of samples or bytes have been written to
the mmap ring buffer. In this case, POLL_IN is indicated.
The other way is by use of the PERF_EVENT_IOC_REFRESH ioctl. This
ioctl adds to a counter that decrements each time the event overflows.
When nonzero, POLL_IN is indicated, but once the counter reaches 0
POLL_HUP is indicated and the underlying event is disabled.
Refreshing an event group leader refreshes all siblings and refreshing
with a parameter of 0 currently enables infinite refreshes; these
behaviors are unsupported and should not be relied on.
Starting with Linux 3.18, POLL_HUP is indicated if the event being mon-
itored is attached to a different process and that process exits.
rdpmc instruction
Starting with Linux 3.4 on x86, you can use the rdpmc instruction to
get low-latency reads without having to enter the kernel. Note that
using rdpmc is not necessarily faster than other methods for reading
event values.
Support for this can be detected with the cap_usr_rdpmc field in the
mmap page; documentation on how to calculate event values can be found
in that section.
Originally, when rdpmc support was enabled, any process (not just ones
with an active perf event) could use the rdpmc instruction to access
the counters. Starting with Linux 4.0, rdpmc support is only allowed
if an event is currently enabled in a process's context. To restore
the old behavior, write the value 2 to /sys/devices/cpu/rdpmc.
perf_event ioctl calls
Various ioctls act on perf_event_open() file descriptors:
PERF_EVENT_IOC_ENABLE
This enables the individual event or event group specified by
the file descriptor argument.
If the PERF_IOC_FLAG_GROUP bit is set in the ioctl argument,
then all events in a group are enabled, even if the event 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. An overflow notification with
POLL_IN set will happen on each overflow until the count reaches
0; when that happens a notification with POLL_HUP set is sent
and the event is disabled. Using an argument of 0 is considered
undefined behavior.
PERF_EVENT_IOC_RESET
Reset the event count specified by the file descriptor argument
to zero. This resets only the counts; there is no way to reset
the multiplexing time_enabled or time_running values.
If the PERF_IOC_FLAG_GROUP bit is set in the ioctl argument,
then all events in a group are reset, even if the event 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.
PERF_EVENT_IOC_SET_BPF (since Linux 4.1)
This allows attaching a Berkeley Packet Filter (BPF) program to
an existing kprobe tracepoint event. You need CAP_SYS_ADMIN
privileges to use this ioctl.
The argument is a BPF program file descriptor that was created
by a previous bpf(2) system call.
Using prctl(2)
A process can enable or disable all the event groups that are attached
to it using the prctl(2) PR_TASK_PERF_EVENTS_ENABLE and
PR_TASK_PERF_EVENTS_DISABLE operations. This applies to all counters
on the calling process, whether created by this process or by another,
and does not affect any counters that this process has created on other
processes. It enables or disables only the group leaders, not any
other members in the groups.
perf_event related configuration files
Files in /proc/sys/kernel/
/proc/sys/kernel/perf_event_paranoid
The perf_event_paranoid file can be set to restrict access
to the performance counters.
2 allow only user-space measurements (default since Linux
4.6).
1 allow both kernel and user measurements (default before
Linux 4.6).
0 allow access to CPU-specific data but not raw tracepoint
samples.
-1 no restrictions.
The existence of the perf_event_paranoid file is the 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_max_stack
This file sets the maximum depth of stack frame entries
reported when generating a call trace.
/proc/sys/kernel/perf_event_mlock_kb
Maximum number of pages an unprivileged user can mlock(2).
The default is 516 (kB).
Files in /sys/bus/event_source/devices/
Since Linux 2.6.34, the kernel supports having multiple PMUs 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/cpu/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.
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 behav-
ior.
/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.
EBUSY (since Linux 4.1)
Returned if another event already has exclusive access to the
PMU.
EFAULT Returned if the attr pointer points at an invalid memory
address.
EINVAL Returned if the specified event is invalid. There are many 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-process limit on the number of
open file descriptors will be reached, and no more events can be
created.
ENODEV Returned when the event involves a feature not supported by the
current CPU.
ENOENT Returned if the type setting is not valid. This error is also
returned for some unsupported generic events.
ENOSPC Prior to Linux 3.3, if there was not enough room for the event,
ENOSPC was returned. In Linux 3.3, this was changed to EINVAL.
ENOSPC is still returned if you try to add more breakpoint
events than supported by the hardware.
ENOSYS Returned if PERF_SAMPLE_STACK_USER is set in sample_type and it
is not supported by hardware.
EOPNOTSUPP
Returned if an event requiring a specific hardware feature is
requested but there is no hardware support. This includes
requesting low-skid events if not supported, branch tracing if
it is not available, sampling if no PMU interrupt is available,
and branch stacks for software events.
EOVERFLOW (since Linux 4.8)
Returned if PERF_SAMPLE_CALLCHAIN is requested and sam-
ple_max_stack is larger than the maximum specified in
/proc/sys/kernel/perf_event_max_stack.
EPERM Returned on many (but not all) architectures when an unsupported
exclude_hv, exclude_idle, exclude_user, or exclude_kernel 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.
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
perf(1), fcntl(2), mmap(2), open(2), prctl(2), read(2)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2018-02-02 PERF_EVENT_OPEN(2)
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