perf_event_open — set up performance monitoring
#include <linux/perf_event.h> #include <linux/hw_breakpoint.h>
int
perf_event_open( |
struct perf_event_attr *attr, |
| pid_t pid, | |
| int cpu, | |
| int group_fd, | |
unsigned long flags); |
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Note |
|---|---|
| There is no glibc wrapper for this system call; see NOTES. |
Given a list of parameters, perf_event_open() returns a file
descriptor, for use in subsequent system calls (read(2), mmap(2), prctl(2), fcntl(2), etc.).
A call to perf_event_open()
creates a file descriptor that allows measuring performance
information. Each file descriptor corresponds to one event
that is measured; these can be grouped together to measure
multiple events simultaneously.
Events can be enabled and disabled in two ways: via ioctl(2) and via prctl(2). When an event is disabled it does not count or generate overflows but does continue to exist and maintain its count value.
Events come in two flavors: counting and sampled. A
counting event is
one that is used for counting the aggregate number of events
that occur. In general, counting event results are gathered
with a read(2) call. A sampling event periodically
writes measurements to a buffer that can then be accessed via
mmap(2).
The argument pid
allows events to be attached to processes in various ways.
If pid is 0,
measurements happen on the current thread, if pid is greater than 0, the
process indicated by pid is measured, and if
pid is −1,
all processes are counted.
The cpu argument
allows measurements to be specific to a CPU. If cpu is greater than or equal
to 0, measurements are restricted to the specified CPU; if
cpu is −1,
the events are measured on all CPUs.
Note that the combination of pid == −1 and
cpu == −1 is
not valid.
A pid > 0 and
cpu == −1
setting measures per-process and follows that process to
whatever CPU the process gets scheduled to. Per-process
events can be created by any user.
A pid ==
−1 and cpu
>= 0 setting is per-CPU and measures all processes on
the specified CPU. Per-CPU events need the CAP_SYS_ADMIN capability or a
/proc/sys/kernel/perf_event_paranoid
value of less than 1.
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 fd 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), etc., with each other, since they have counted
events for the same set of executed instructions.
The flags
argument is formed by ORing together zero or more of the
following values:
PERF_FLAG_FD_NO_GROUPThis 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_OUTPUTThis flag re-routes 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 grain control
(CPUs, memory, etc.). In this mode, the event is
measured only if the thread running on the monitored
CPU belongs to the designated container (cgroup). The
cgroup is identified by passing a file descriptor
opened on its directory in the cgroupfs filesystem.
For instance, if the cgroup to monitor is called
test, then
a file descriptor opened on /dev/cgroup/test (assuming cgroupfs
is mounted on /dev/cgroup) must be passed as the
pid
parameter. cgroup monitoring is available only for
system-wide events and may therefore require extra
permissions.
The perf_event_attr structure provides detailed configuration information for the event being created.
struct perf_event_attr { __u32 type; /* Type of event */ __u32 size; /* Size of attribute structure */ __u64 config; /* Type-specific configuration */ union { __u64 sample_period; /* Period of sampling */ __u64 sample_freq; /* Frequency of sampling */ }; __u64 sample_type; /* Specifies values included in sample */ __u64 read_format; /* Specifies values returned in read */ __u64 disabled : 1, /* off by default */ inherit : 1, /* children inherit it */ pinned : 1, /* must always be on PMU */ exclusive : 1, /* only group on PMU */ exclude_user : 1, /* don't count user */ exclude_kernel : 1, /* don't count kernel */ exclude_hv : 1, /* don't count hypervisor */ exclude_idle : 1, /* don't count when idle */ mmap : 1, /* include mmap data */ comm : 1, /* include comm data */ freq : 1, /* use freq, not period */ inherit_stat : 1, /* per task counts */ enable_on_exec : 1, /* next exec enables */ task : 1, /* trace fork/exit */ watermark : 1, /* wakeup_watermark */ precise_ip : 2, /* skid constraint */ mmap_data : 1, /* non-exec mmap data */ sample_id_all : 1, /* sample_type all events */ exclude_host : 1, /* don't count in host */ exclude_guest : 1, /* don't count in guest */ exclude_callchain_kernel : 1, /* exclude kernel callchains */ exclude_callchain_user : 1, /* exclude user callchains */ __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:
typeThis field specifies the overall event type. It has one of the following values:
PERF_TYPE_HARDWAREThis indicates one of the "generalized" hardware events provided by the kernel. See the
configfield definition for more details.PERF_TYPE_SOFTWAREThis indicates one of the software-defined events provided by the kernel (even if no hardware support is available).
PERF_TYPE_TRACEPOINTThis indicates a tracepoint provided by the kernel tracepoint infrastructure.
PERF_TYPE_HW_CACHEThis indicates a hardware cache event. This has a special encoding, described in the
configfield definition.PERF_TYPE_RAWThis indicates a "raw" implementation-specific event in the
configfield.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 multiple PMUs. To enable this, a value exported by the kernel can be used in thetypefield to indicate which PMU to use. The value to use can be found in the sysfs filesystem: there is a subdirectory per PMU instance under/sys/bus/event_source/devices. In each sub-directory there is atypefile whose content is an integer that can be used in thetypefield. For instance,/sys/bus/event_source/devices/cpu/typecontains the value for the core CPU PMU, which is usually 4.
sizeThe size of the perf_event_attr structure for forward/backward compatibility. Set this using sizeof(struct perf_event_attr) to allow the kernel to see the struct size at the time of compilation.
The related define PERF_ATTR_SIZE_VER0 is set to 64;
this was the size of the first published struct.
PERF_ATTR_SIZE_VER1 is
72, corresponding to the addition of breakpoints in
Linux 2.6.33. PERF_ATTR_SIZE_VER2 is 80
corresponding to the addition of branch sampling in
Linux 3.4. PERF_ATR_SIZE_VER3 is 96
corresponding to the addition of sample_regs_user and
sample_stack_user in
Linux 3.7.
configThis 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-specific (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
dependent on the value of the previously described
type field.
What follows are various possible settings for
config
separated out by type.
If type
is PERF_TYPE_HARDWARE,
we are measuring one of the generalized hardware CPU
events. Not all of these are available on all
platforms. Set config to one of the
following:
PERF_COUNT_HW_CPU_CYCLESTotal cycles. Be wary of what happens during CPU frequency scaling.
PERF_COUNT_HW_INSTRUCTIONSRetired instructions. Be careful, these can be affected by various issues, most notably hardware interrupt counts.
PERF_COUNT_HW_CACHE_REFERENCESCache accesses. Usually this indicates Last Level Cache accesses but this may vary depending on your CPU. This may include prefetches and coherency messages; again this depends on the design of your CPU.
PERF_COUNT_HW_CACHE_MISSESCache misses. Usually this indicates Last Level Cache misses; this is intended to be used in conjunction with the
PERF_COUNT_HW_CACHE_REFERENCESevent to calculate cache miss rates.PERF_COUNT_HW_BRANCH_INSTRUCTIONSRetired branch instructions. Prior to Linux 2.6.34, this used the wrong event on AMD processors.
PERF_COUNT_HW_BRANCH_MISSESMispredicted branch instructions.
PERF_COUNT_HW_BUS_CYCLESBus 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_CLOCKThis reports the CPU clock, a high-resolution per-CPU timer.
PERF_COUNT_SW_TASK_CLOCKThis reports a clock count specific to the task that is running.
PERF_COUNT_SW_PAGE_FAULTSThis reports the number of page faults.
PERF_COUNT_SW_CONTEXT_SWITCHESThis 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_MIGRATIONSThis reports the number of times the process has migrated to a new CPU.
PERF_COUNT_SW_PAGE_FAULTS_MINThis counts the number of minor page faults. These did not require disk I/O to handle.
PERF_COUNT_SW_PAGE_FAULTS_MAJThis counts the number of major page faults. These required disk I/O to handle.
PERF_COUNT_SW_ALIGNMENT_FAULTS(Since Linux 2.6.33)This counts the number of alignment faults. These happen when unaligned memory accesses happen; the kernel can handle these but it reduces performance. This happens only on some architectures (never on x86).
PERF_COUNT_SW_EMULATION_FAULTS(Since Linux 2.6.33)This counts the number of emulation faults. The kernel sometimes traps on unimplemented instructions and emulates them for user space. This can negatively impact performance.
If
typeisPERF_TYPE_TRACEPOINT, then we are measuring kernel tracepoints. The value to use inconfigcan be obtained from under debugfstracing/events/*/*/idif ftrace is enabled in the kernel.
If
typeisPERF_TYPE_HW_CACHE, then we are measuring a hardware CPU cache event. To calculate the appropriateconfigvalue 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_idis one of:
PERF_COUNT_HW_CACHE_L1Dfor measuring Level 1 Data Cache
PERF_COUNT_HW_CACHE_L1Ifor measuring Level 1 Instruction Cache
PERF_COUNT_HW_CACHE_LLfor measuring Last-Level Cache
PERF_COUNT_HW_CACHE_DTLBfor measuring the Data TLB
PERF_COUNT_HW_CACHE_ITLBfor measuring the Instruction TLB
PERF_COUNT_HW_CACHE_BPUfor measuring the branch prediction unit
PERF_COUNT_HW_CACHE_NODE(Since Linux 3.0)for measuring local memory accesses
and
perf_hw_cache_op_idis one of
PERF_COUNT_HW_CACHE_OP_READfor read accesses
PERF_COUNT_HW_CACHE_OP_WRITEfor write accesses
PERF_COUNT_HW_CACHE_OP_PREFETCHfor prefetch accesses
and
perf_hw_cache_op_result_idis one of
PERF_COUNT_HW_CACHE_RESULT_ACCESSto measure accesses
PERF_COUNT_HW_CACHE_RESULT_MISSto measure misses
If
typeisPERF_TYPE_RAW, then a custom "raw"configvalue is needed. Most CPUs support events that are not covered by the "generalized" events. These are implementation defined; see your CPU manual (for example the Intel Volume 3B documentation or the AMD BIOS and Kernel Developer Guide). The libpfm4 library can be used to translate from the name in the architectural manuals to the raw hex valueperf_event_open() expects in this field.If
typeisPERF_TYPE_BREAKPOINT, then leaveconfigset to zero. Its parameters are set in other places.
sample_period,
sample_freqA "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_typeThe 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_IPRecords instruction pointer.
PERF_SAMPLE_TIDRecords the process and thread IDs.
PERF_SAMPLE_TIMERecords a timestamp.
PERF_SAMPLE_ADDRRecords an address, if applicable.
PERF_SAMPLE_READRecord counter values for all events in a group, not just the group leader.
PERF_SAMPLE_CALLCHAINRecords the callchain (stack backtrace).
PERF_SAMPLE_IDRecords a unique ID for the opened event's group leader.
PERF_SAMPLE_CPURecords CPU number.
PERF_SAMPLE_PERIODRecords the current sampling period.
PERF_SAMPLE_STREAM_IDRecords a unique ID for the opened event. Unlike
PERF_SAMPLE_IDthe actual ID is returned, not the group leader. This ID is the same as the one returned by PERF_FORMAT_ID.PERF_SAMPLE_RAWRecords 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_typefield for how to filter which branches are reported.PERF_SAMPLE_REGS_USER(Since Linux 3.7)Records the current user-level CPU register state (the values in the process before the kernel was called).
PERF_SAMPLE_STACK_USER(Since Linux 3.7)Records the user level stack, allowing stack unwinding.
PERF_SAMPLE_WEIGHT(Since Linux 3.10)Records a hardware provided weight value that expresses how costly the sampled event was. This allows the hardware to highlight expensive events in a profile.
PERF_SAMPLE_DATA_SRC(Since Linux 3.10)Records the data source: where in the memory hierarchy the data associated with the sampled instruction came from. This is only available if the underlying hardware supports this feature.
read_formatThis field specifies the format of the data
returned by read(2) on a
perf_event_open() file
descriptor.
PERF_FORMAT_TOTAL_TIME_ENABLEDAdds the 64-bit
time_enabledfield. This can be used to calculate estimated totals if the PMU is overcommitted and multiplexing is happening.PERF_FORMAT_TOTAL_TIME_RUNNINGAdds the 64-bit
time_runningfield. This can be used to calculate estimated totals if the PMU is overcommitted and multiplexing is happening.PERF_FORMAT_IDAdds a 64-bit unique value that corresponds to the event group.
PERF_FORMAT_GROUPAllows all counter values in an event group to be read with one read.
disabledThe disabled bit
specifies whether the counter starts out disabled or
enabled. If disabled, the event can later be enabled
by ioctl(2), prctl(2), or
enable_on_exec.
inheritThe inherit bit specifies
that this counter should count events of child tasks
as well as the task specified. This applies only to
new children, not to any existing children at the
time the counter is created (nor to any new children
of existing children).
Inherit does not work for some combinations of
read_formats, such as
PERF_FORMAT_GROUP.
pinnedThe pinned bit specifies
that the counter should always be on the CPU if at
all possible. It applies only to hardware counters
and only to group leaders. If a pinned counter cannot
be put onto the CPU (e.g., because there are not
enough hardware counters or because of a conflict
with some other event), then the counter goes into an
'error' state, where reads return end-of-file (i.e.,
read(2) returns 0)
until the counter is subsequently enabled or
disabled.
exclusiveThe 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.
exclude_userIf this bit is set, the count excludes events that happen in user space.
exclude_kernelIf this bit is set, the count excludes events that happen in kernel-space.
exclude_hvIf 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_idleIf set, don't count when the CPU is idle.
mmapThe mmap
bit enables recording of exec mmap events.
commThe comm
bit enables tracking of process command name as
modified by the exec(2) and
prctl(PR_SET_NAME)
system calls. Unfortunately for tools, there is no
way to distinguish one system call versus the
other.
freqIf this bit is set, then sample_frequency not
sample_period is used
when setting up the sampling interval.
inherit_statThis bit enables saving of event counts on context
switch for inherited tasks. This is meaningful only
if the inherit field is
set.
enable_on_execIf this bit is set, a counter is automatically enabled after a call to exec(2).
taskIf this bit is set, then fork/exit notifications are included in the ring buffer.
watermarkIf 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_IPcan have arbitrary skid.- 1 -
SAMPLE_IPmust have constant skid.- 2 -
SAMPLE_IPrequested to have 0 skid.- 3 -
SAMPLE_IPmust have 0 skid. See alsoPERF_RECORD_MISC_EXACT_IP.
mmap_data (Since Linux
2.6.36)The counterpart of the mmap field, but
enables including data mmap events in the
ring-buffer.
sample_id_all (Since
Linux 2.6.38)If set, then TID, TIME, ID, CPU, and STREAM_ID can
additionally be included in non-PERF_RECORD_SAMPLEs if the
corresponding sample_type is
selected.
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_watermarkThis 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_EMPTYNo breakpoint.
HW_BREAKPOINT_RCount when we read the memory location.
HW_BREAKPOINT_WCount when we write the memory location.
HW_BREAKPOINT_RWCount when we read or write the memory location.
HW_BREAKPOINT_XCount when we execute code at the memory location.
The values can be combined via a bitwise or, but the combination of
HW_BREAKPOINT_RorHW_BREAKPOINT_WwithHW_BREAKPOINT_Xis not allowed.
bp_addr (Since Linux
2.6.33)bp_addr
address of the breakpoint. For execution breakpoints
this is the memory address of the instruction of
interest; for read and write breakpoints it is the
memory address of the memory location of
interest.
config1 (Since Linux
2.6.39)config1
is used for setting events that need an extra
register or otherwise do not fit in the regular
config field. Raw OFFCORE_EVENTS on
Nehalem/Westmere/SandyBridge use this field on 3.3
and later kernels.
bp_len (Since Linux
2.6.33)bp_len
is the length of the breakpoint being measured if
type is
PERF_TYPE_BREAKPOINT.
Options are HW_BREAKPOINT_LEN_1, 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_USERBranch target is in user space.
PERF_SAMPLE_BRANCH_KERNELBranch target is in kernel space.
PERF_SAMPLE_BRANCH_HVBranch target is in hypervisor.
PERF_SAMPLE_BRANCH_PLM_ALLA 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_ANYAny branch type.
PERF_SAMPLE_BRANCH_ANY_CALLAny call branch.
PERF_SAMPLE_BRANCH_ANY_RETURNAny return branch.
PERF_SAMPLE_BRANCH_IND_CALLIndirect 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 bitmask defines the set of user CPU registers
to dump on samples. The layout of the register mask
is architecture specific and described in the kernel
header arch/ARCH/include/uapi/asm/perf_regs.h.
sample_stack_user
(Since Linux 3.7)This defines the size of the user stack to dump if
PERF_SAMPLE_STACK_USER
is specified.
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:
nrThe number of events in this file descriptor. Only
available if PERF_FORMAT_GROUP was
specified.
time_enabled, time_runningTotal time the event was enabled and running.
Normally these are the same. If more events are
started than 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.
valueAn unsigned 64-bit value containing the counter result.
idA globally unique value for this particular event,
only there if PERF_FORMAT_ID was specified in
read_format.
When using perf_event_open() in sampled mode,
asynchronous events (like counter overflow or PROT_EXEC mmap tracking) are logged into
a ring-buffer. This ring-buffer is created and accessed
through mmap(2).
The mmap size should be 1+2^n pages, where the first page is a metadata page (struct perf_event_mmap_page) that contains various bits of information such as where the ring-buffer head is.
Before kernel 2.6.39, there is a bug that means you must allocate 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; __u64 cap_usr_time : 1, cap_usr_rdpmc : 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 looks at the fields in the perf_event_mmap_page
structure in more detail:
versionVersion number of this structure.
compat_versionThe lowest version this is compatible with.
lockA seqlock for synchronization.
indexA unique hardware counter identifier.
offsetAdd this to hardware counter value??
time_enabledTime the event was active.
time_runningTime the event was running.
cap_usr_timeUser time capability.
cap_usr_rdpmcIf 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; s64 pmc = 0; 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; pmc = rdpmc(idx − 1); } barrier(); } while (pc−>lock != seq);
pmc_widthIf 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_offsetIf cap_usr_time, these
fields can be used to compute the time delta since
time_enabled (in nanoseconds) using rdtsc or
similar.
u64 quot, rem;
u64 delta;
quot = (cyc >> time_shift);
rem = cyc & ((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;
data_headThis points to the head of the data section. The value continuously increases, it does not wrap. The value needs to be manually wrapped by the size of the mmap buffer before accessing the samples.
On SMP-capable platforms, after reading the data_head value, user space should issue an rmb().
data_tail;When the mapping is PROT_WRITE, the data_tail value
should be written by user space to reflect the last
read data. In this case the kernel will not
over-write 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.
sizeThis indicates the size of the record.
miscThe misc
field contains additional information about the
sample.
The CPU mode can be determined from this value by
masking with PERF_RECORD_MISC_CPUMODE_MASK and
looking for one of the following (note these are not
bit masks, only one can be set at a time):
PERF_RECORD_MISC_CPUMODE_UNKNOWNUnknown CPU mode.
PERF_RECORD_MISC_KERNELSample happened in the kernel.
PERF_RECORD_MISC_USERSample happened in user code.
PERF_RECORD_MISC_HYPERVISORSample happened in the hypervisor.
PERF_RECORD_MISC_GUEST_KERNELSample happened in the guest kernel.
PERF_RECORD_MISC_GUEST_USERSample happened in guest user code.
In addition, one of the following bits can be set:
PERF_RECORD_MISC_MMAP_DATAThis is set when the mapping is not executable; otherwise the mapping is executable.
PERF_RECORD_MISC_EXACT_IPThis indicates that the content of
PERF_SAMPLE_IPpoints to the actual instruction that triggered the event. See alsoperf_event_attr.precise_ip.PERF_RECORD_MISC_EXT_RESERVEDThis indicates there is extended data available (currently not used).
typeThe type
value is one of the below. The values in the
corresponding record (that follows the header) depend
on the type
selected as shown.
PERF_RECORD_MMAPThe MMAP events record the
PROT_EXECmappings 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_LOSTThis record indicates when events are lost.
struct { struct perf_event_header header;u64 id;u64 lost;};
idis the unique event ID for the samples that were lost.
lostis the number of events that were lost.
PERF_RECORD_COMMThis record indicates a change in the process name.
struct { struct perf_event_header header;u32 pid,tid;char comm[];}; PERF_RECORD_EXITThis record indicates a process exit event.
struct { struct perf_event_header header;u32 pid,ppid;u32 tid,ptid;u64 time;}; PERF_RECORD_THROTTLE,PERF_RECORD_UNTHROTTLEThis record indicates a throttle/unthrottle event.
struct { struct perf_event_header header;u64 time;u64 id;u64 stream_id;}; PERF_RECORD_FORKThis record indicates a fork event.
struct { struct perf_event_header header;u32 pid,ppid;u32 tid,ptid;u64 time;}; PERF_RECORD_READThis record indicates a read event.
struct { struct perf_event_header header;u32 pid,tid;struct read_format values;}; PERF_RECORD_SAMPLEThis record indicates a sample.
struct { struct perf_event_header header; 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 */ };
ipIf
PERF_SAMPLE_IPis enabled, then a 64-bit instruction pointer value is included.pid,tidIf
PERF_SAMPLE_TIDis enabled, then a 32-bit process ID and 32-bit thread ID are included.timeIf
PERF_SAMPLE_TIMEis 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.addrIf
PERF_SAMPLE_ADDRis 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.idIf
PERF_SAMPLE_IDis 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 byPERF_FORMAT_ID.stream_idIf
PERF_SAMPLE_STREAM_IDis enabled, a 64-bit unique ID is included. UnlikePERF_SAMPLE_IDthe actual ID is returned, not the group leader. This ID is the same as the one returned byPERF_FORMAT_ID.cpu,resIf
PERF_SAMPLE_CPUis enabled, this is a 32-bit value indicating which CPU was being used, in addition to a reserved (unused) 32-bit value.periodIf
PERF_SAMPLE_PERIODis enabled, a 64-bit value indicating the current sampling period is written.vIf
PERF_SAMPLE_READis enabled, a structure of type read_format is included which has values for all events in the event group. The values included depend on theread_formatvalue used atperf_event_open() time.nr,ips[nr]If
PERF_SAMPLE_CALLCHAINis enabled, then a 64-bit number is included which indicates how many following 64-bit instruction pointers will follow. This is the current callchain.size,data[size]If
PERF_SAMPLE_RAWis enabled, then a 32-bit value indicating size is included followed by an array of 8-bit values of length size. The values are padded with 0 to have 64-bit alignment.This RAW record data is opaque with respect to the ABI. The ABI doesn't make any promises with respect to the stability of its content, it may vary depending on event, hardware, and kernel version.
bnr,lbr[bnr]If
PERF_SAMPLE_BRANCH_STACKis enabled, then a 64-bit value indicating the number of records is included, followed bybnrperf_branch_entrystructures which each include the fields:
fromThis indicates the source instruction (may not be a branch).
toThe branch target.
mispredThe branch target was mispredicted.
predictedThe branch target was predicted.
in_tx(Since Linux 3.11)The branch was in a transactional memory transaction.
abort(Since Linux 3.11)The branch was in an aborted transactional memory transaction.
The entries are from most to least recent, so the first entry has the most recent branch.
Support for
mispredandpredictedis optional; if not supported, both values will be 0.The type of branches recorded is specified by the
branch_sample_typefield.abi,regs[weight(mask)]If
PERF_SAMPLE_REGS_USERis enabled, then the user CPU registers are recorded.The
abifield is one ofPERF_SAMPLE_REGS_ABI_NONE,PERF_SAMPLE_REGS_ABI_32orPERF_SAMPLE_REGS_ABI_64.The
regsfield is an array of the CPU registers that were specified by thesample_regs_userattr field. The number of values is the number of bits set in thesample_regs_userbitmask.size,data[size],dyn_sizeIf
PERF_SAMPLE_STACK_USERis enabled, then record the user stack to enable backtracing.sizeis the size requested by the user instack_user_sizeor else the maximum record size.datais the stack data.dyn_sizeis the amount of data actually dumped (can be less thansize).weightIf
PERF_SAMPLE_WEIGHTis 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_srcIf
PERF_SAMPLE_DATA_SRCis enabled, then a 64 bit value is recorded that is made up of the following fields:
mem_opType of opcode, a bitwise combination of:
PERF_MEM_OP_NANot available
PERF_MEM_OP_LOADLoad instruction
PERF_MEM_OP_STOREStore instruction
PERF_MEM_OP_PFETCHPrefetch
PERF_MEM_OP_EXECExecutable code
mem_lvlMemory hierarchy level hit or miss, a bitwise combination of:
PERF_MEM_LVL_NANot available
PERF_MEM_LVL_HITHit
PERF_MEM_LVL_MISSMiss
PERF_MEM_LVL_L1Level 1 cache
PERF_MEM_LVL_LFBLine fill buffer
PERF_MEM_LVL_L2Level 2 cache
PERF_MEM_LVL_L3Level 3 cache
PERF_MEM_LVL_LOC_RAMLocal DRAM
PERF_MEM_LVL_REM_RAM1Remote DRAM 1 hop
PERF_MEM_LVL_REM_RAM2Remote DRAM 2 hops
PERF_MEM_LVL_REM_CCE1Remote cache 1 hop
PERF_MEM_LVL_REM_CCE2Remote cache 2 hops
PERF_MEM_LVL_IOI/O memory
PERF_MEM_LVL_UNCUncached memory
mem_snoopSnoop mode, a bitwise combination of:
PERF_MEM_SNOOP_NANot available
PERF_MEM_SNOOP_NONENo snoop
PERF_MEM_SNOOP_HITSnoop hit
PERF_MEM_SNOOP_MISSSnoop miss
PERF_MEM_SNOOP_HITMSnoop hit modified
mem_lockLock instruction, a bitwise combination of:
PERF_MEM_LOCK_NANot available
PERF_MEM_LOCK_LOCKEDLocked transaction
mem_dtlbTLB access hit or miss, a bitwise combination of:
PERF_MEM_TLB_NANot available
PERF_MEM_TLB_HITHit
PERF_MEM_TLB_MISSMiss
PERF_MEM_TLB_L1Level 1 TLB
PERF_MEM_TLB_L2Level 2 TLB
PERF_MEM_TLB_WKHardware walker
PERF_MEM_TLB_OSOS fault handler
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 non-zero 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 non-zero, 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.
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Note |
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On newer kernels (definitely noticed with 3.2) a
signal is provided for every overflow, even if
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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.
Various ioctls act on perf_event_open() file descriptors
PERF_EVENT_IOC_ENABLEEnables the individual event or event group specified by the file descriptor argument.
If the PERF_IOC_FLAG_GROUP bit is set in
the ioctl argument, then all events in a group are
enabled, even if the event specified is not the group
leader (but see BUGS).
PERF_EVENT_IOC_DISABLEDisables the individual counter or event group specified by the file descriptor argument.
Enabling or disabling the leader of a group enables or disables the entire group; that is, while the group leader is disabled, none of the counters in the group will count. Enabling or disabling a member of a group other than the leader affects only that counter; disabling a non-leader stops that counter from counting but doesn't affect any other counter.
If the PERF_IOC_FLAG_GROUP bit is set in
the ioctl argument, then all events in a group are
disabled, even if the event specified is not the
group leader (but see BUGS).
PERF_EVENT_IOC_REFRESHNon-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 happen on each
overflow until the count reaches 0; when that happens
a signal with POLL_HUP set is sent and the event is
disabled. Using an argument of 0 is considered
undefined behavior.
PERF_EVENT_IOC_RESETReset the event count specified by the file
descriptor argument to zero. This resets only the
counts; there is no way to reset the multiplexing
time_enabled
or time_running
values.
If the PERF_IOC_FLAG_GROUP bit is set in
the ioctl argument, then all events in a group are
reset, even if the event specified is not the group
leader (but see BUGS).
PERF_EVENT_IOC_PERIODIOC_PERIOD is the command to update the period; it does not update the current period but instead defers until next.
The argument is a pointer to a 64-bit value containing the desired new period.
PERF_EVENT_IOC_SET_OUTPUTThis tells the kernel to report event notifications to the specified file descriptor rather than the default one. The file descriptors must all be on the same CPU.
The argument specifies the desired file descriptor, or −1 if output should be ignored.
PERF_EVENT_IOC_SET_FILTER (Since
Linux 2.6.33)This adds an ftrace filter to this event.
The argument is a pointer to the desired ftrace filter.
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 current
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.
Files in /proc/sys/kernel/
/proc/sys/kernel/perf_event_paranoidThe
perf_event_paranoidfile can be set to restrict access to the performance counters.
2only allow user-space measurements.
1allow both kernel and user measurements (default).
0allow access to CPU-specific data but not raw tracepoint samples.
- −1
no restrictions.
The existence of the
perf_event_paranoidfile is the official method for determining if a kernel supportsperf_event_open()./proc/sys/kernel/perf_event_max_sample_rateThis 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_kbMaximum number of pages an unprivileged user can mlock (2) . The default is 516 (kB).
Files in /sys/bus/event_source/devices/
Since Linux 2.6.34 the kernel supports having multiple PMUs available for monitoring. Information on how to program these PMUs can be found under
/sys/bus/event_source/devices/. Each subdirectory corresponds to a different PMU.
/sys/bus/event_source/devices/*/type(Since Linux 2.6.38)This contains an integer that can be used in the
typefield of perf_event_attr to indicate 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 performance counter registers is allowed via the rdpmc instruction. This can be disabled by echoing 0 to the file.
/sys/bus/event_source/devices/*/format/(Since Linux 3.4)This sub-directory contains information on the architecture-specific sub-fields available for programming the various
configfields 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
eventmay contain the valueconfig1:1,6-10,44which 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 sub-directory contains files with pre-defined events. The contents are strings describing the event settings expressed in terms of the fields found in the previously mentioned
./format/directory. These are not necessarily complete lists of all events supported by a PMU, but usually a subset of events deemed useful or interesting.The content of each file is a list of attribute names separated by commas. Each entry has an optional value (either hex or decimal). If no value is specified than 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/*/ueventThis file is the standard kernel device interface for injecting hotplug events.
/sys/bus/event_source/devices/*/cpumask(Since Linux 3.7)The
cpumaskfile contains a comma-separated list of integers that indicate a representative CPU number for each socket (package) on the motherboard. This is needed when setting up uncore or northbridge events, as those PMUs present socket-wide events.
perf_event_open() returns
the new file descriptor, or −1 if an error occurred (in
which case, errno is set
appropriately).
Returned if the specified event is not available.
Prior to Linux 3.3, if there was not enough room for the event, ENOSPC was returned. Linus did not like this, and this was changed to EINVAL. ENOSPC is still returned if you try to read results into too small of a buffer.
perf_event_open() was
introduced in Linux 2.6.31 but was called perf_counter_open(). It was renamed in
Linux 2.6.32.
This perf_event_open()
system call Linux- specific and should not be used in
programs intended to be portable.
Glibc does not provide a wrapper for this system call; call it using syscall(2). See the example below.
The official way of knowing if perf_event_open() support is enabled is
checking for the existence of the file /proc/sys/kernel/perf_event_paranoid.
The F_SETOWN_EX option to
fcntl(2) is needed to
properly get overflow signals in threads. This was introduced
in Linux 2.6.32.
Prior to Linux 2.6.33 (at least for x86) the kernel did
not check if events could be scheduled together until read
time. The same happens on all known kernels if the NMI
watchdog is enabled. This means to see if a given set of
events works you have to perf_event_open(), start, then read before
you know for sure you can get valid measurements.
Prior to Linux 2.6.34 event constraints were not enforced by the kernel. In that case, some events would silently return "0" if the kernel scheduled them in an improper counter slot.
Prior to Linux 2.6.34 there was a bug when multiplexing where the wrong results could be returned.
Kernels from Linux 2.6.35 to Linux 2.6.39 can quickly crash the kernel if "inherit" is enabled and many threads are started.
Prior to Linux 2.6.35, PERF_FORMAT_GROUP did not work with
attached processes.
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 non-zero 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.
Always double-check your results! Various generalized events have had wrong values. For example, retired branches measured the wrong thing on AMD machines until Linux 2.6.35.
The following is a short example that measures the total instruction count of a call to printf(3).
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/ioctl.h>
#include <linux/perf_event.h>
#include <asm/unistd.h>
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);
}
This page is part of release 3.54 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/.
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Copyright (c) 2012, Vincent Weaver %%%LICENSE_START(GPLv2+_DOC_FULL) This is free documentation; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. The GNU General Public License's references to "object code" and "executables" are to be interpreted as the output of any document formatting or typesetting system, including intermediate and printed output. This manual is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this manual; if not, see <http://www.gnu.org/licenses/>. %%%LICENSE_END This document is based on the perf_event.h header file, the tools/perf/design.txt file, and a lot of bitter experience. |