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NAME | SYNOPSIS | DESCRIPTION | QUICKSTART | PERF RECORD | PERF SCRIPT | PERF REPORT | PERF INJECT | PEBS VIA INTEL PT | XED | SEE ALSO | COLOPHON |
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PERF-INTEL-PT(1) perf Manual PERF-INTEL-PT(1)
perf-intel-pt - Support for Intel Processor Trace within perf
tools
perf record -e intel_pt//
Intel Processor Trace (Intel PT) is an extension of Intel
Architecture that collects information about software execution
such as control flow, execution modes and timings and formats it
into highly compressed binary packets. Technical details are
documented in the Intel 64 and IA-32 Architectures Software
Developer Manuals, Chapter 36 Intel Processor Trace.
Intel PT is first supported in Intel Core M and 5th generation
Intel Core processors that are based on the Intel
micro-architecture code name Broadwell.
Trace data is collected by perf record and stored within the
perf.data file. See below for options to perf record.
Trace data must be decoded which involves walking the object code
and matching the trace data packets. For example a TNT packet
only tells whether a conditional branch was taken or not taken,
so to make use of that packet the decoder must know precisely
which instruction was being executed.
Decoding is done on-the-fly. The decoder outputs samples in the
same format as samples output by perf hardware events, for
example as though the "instructions" or "branches" events had
been recorded. Presently 3 tools support this: perf script, perf
report and perf inject. See below for more information on using
those tools.
The main distinguishing feature of Intel PT is that the decoder
can determine the exact flow of software execution. Intel PT can
be used to understand why and how did software get to a certain
point, or behave a certain way. The software does not have to be
recompiled, so Intel PT works with debug or release builds,
however the executed images are needed - which makes use in
JIT-compiled environments, or with self-modified code, a
challenge. Also symbols need to be provided to make sense of
addresses.
A limitation of Intel PT is that it produces huge amounts of
trace data (hundreds of megabytes per second per core) which
takes a long time to decode, for example two or three orders of
magnitude longer than it took to collect. Another limitation is
the performance impact of tracing, something that will vary
depending on the use-case and architecture.
It is important to start small. That is because it is easy to
capture vastly more data than can possibly be processed.
The simplest thing to do with Intel PT is userspace profiling of
small programs. Data is captured with perf record e.g. to trace
ls userspace-only:
perf record -e intel_pt//u ls
And profiled with perf report e.g.
perf report
To also trace kernel space presents a problem, namely kernel
self-modifying code. A fairly good kernel image is available in
/proc/kcore but to get an accurate image a copy of /proc/kcore
needs to be made under the same conditions as the data capture.
perf record can make a copy of /proc/kcore if the option --kcore
is used, but access to /proc/kcore is restricted e.g.
sudo perf record -o pt_ls --kcore -e intel_pt// -- ls
which will create a directory named pt_ls and put the perf.data
file (named simply data) and copies of /proc/kcore,
/proc/kallsyms and /proc/modules into it. The other tools
understand the directory format, so to use perf report becomes:
sudo perf report -i pt_ls
Because samples are synthesized after-the-fact, the sampling
period can be selected for reporting. e.g. sample every
microsecond
sudo perf report pt_ls --itrace=i1usge
See the sections below for more information about the --itrace
option.
Beware the smaller the period, the more samples that are
produced, and the longer it takes to process them.
Also note that the coarseness of Intel PT timing information will
start to distort the statistical value of the sampling as the
sampling period becomes smaller.
To represent software control flow, "branches" samples are
produced. By default a branch sample is synthesized for every
single branch. To get an idea what data is available you can use
the perf script tool with all itrace sampling options, which will
list all the samples.
perf record -e intel_pt//u ls
perf script --itrace=ibxwpe
An interesting field that is not printed by default is flags
which can be displayed as follows:
perf script --itrace=ibxwpe -F+flags
The flags are "bcrosyiABEx" which stand for branch, call, return,
conditional, system, asynchronous, interrupt, transaction abort,
trace begin, trace end, and in transaction, respectively.
perf script also supports higher level ways to dump instruction
traces:
perf script --insn-trace --xed
Dump all instructions. This requires installing the xed tool (see
XED below) Dumping all instructions in a long trace can be fairly
slow. It is usually better to start with higher level decoding,
like
perf script --call-trace
or
perf script --call-ret-trace
and then select a time range of interest. The time range can then
be examined in detail with
perf script --time starttime,stoptime --insn-trace --xed
While examining the trace it’s also useful to filter on specific
CPUs using the -C option
perf script --time starttime,stoptime --insn-trace --xed -C 1
Dump all instructions in time range on CPU 1.
Another interesting field that is not printed by default is ipc
which can be displayed as follows:
perf script --itrace=be -F+ipc
There are two ways that instructions-per-cycle (IPC) can be
calculated depending on the recording.
If the cyc config term (see config terms section below) was used,
then IPC is calculated using the cycle count from CYC packets,
otherwise MTC packets are used - refer to the mtc config term.
When MTC is used, however, the values are less accurate because
the timing is less accurate.
Because Intel PT does not update the cycle count on every branch
or instruction, the values will often be zero. When there are
values, they will be the number of instructions and number of
cycles since the last update, and thus represent the average IPC
since the last IPC for that event type. Note IPC for "branches"
events is calculated separately from IPC for "instructions"
events.
Also note that the IPC instruction count may or may not include
the current instruction. If the cycle count is associated with an
asynchronous branch (e.g. page fault or interrupt), then the
instruction count does not include the current instruction,
otherwise it does. That is consistent with whether or not that
instruction has retired when the cycle count is updated.
Another note, in the case of "branches" events, non-taken
branches are not presently sampled, so IPC values for them do not
appear e.g. a CYC packet with a TNT packet that starts with a
non-taken branch. To see every possible IPC value, "instructions"
events can be used e.g. --itrace=i0ns
While it is possible to create scripts to analyze the data, an
alternative approach is available to export the data to a sqlite
or postgresql database. Refer to script export-to-sqlite.py or
export-to-postgresql.py for more details, and to script
exported-sql-viewer.py for an example of using the database.
There is also script intel-pt-events.py which provides an example
of how to unpack the raw data for power events and PTWRITE.
As mentioned above, it is easy to capture too much data. One way
to limit the data captured is to use snapshot mode which is
explained further below. Refer to new snapshot option and Intel
PT modes of operation further below.
Another problem that will be experienced is decoder errors. They
can be caused by inability to access the executed image,
self-modified or JIT-ed code, or the inability to match side-band
information (such as context switches and mmaps) which results in
the decoder not knowing what code was executed.
There is also the problem of perf not being able to copy the data
fast enough, resulting in data lost because the buffer was full.
See Buffer handling below for more details.
new event
The Intel PT kernel driver creates a new PMU for Intel PT. PMU
events are selected by providing the PMU name followed by the
"config" separated by slashes. An enhancement has been made to
allow default "config" e.g. the option
-e intel_pt//
will use a default config value. Currently that is the same as
-e intel_pt/tsc,noretcomp=0/
which is the same as
-e intel_pt/tsc=1,noretcomp=0/
Note there are now new config terms - see section config terms
further below.
The config terms are listed in /sys/devices/intel_pt/format. They
are bit fields within the config member of the struct
perf_event_attr which is passed to the kernel by the
perf_event_open system call. They correspond to bit fields in the
IA32_RTIT_CTL MSR. Here is a list of them and their definitions:
$ grep -H . /sys/bus/event_source/devices/intel_pt/format/*
/sys/bus/event_source/devices/intel_pt/format/cyc:config:1
/sys/bus/event_source/devices/intel_pt/format/cyc_thresh:config:19-22
/sys/bus/event_source/devices/intel_pt/format/mtc:config:9
/sys/bus/event_source/devices/intel_pt/format/mtc_period:config:14-17
/sys/bus/event_source/devices/intel_pt/format/noretcomp:config:11
/sys/bus/event_source/devices/intel_pt/format/psb_period:config:24-27
/sys/bus/event_source/devices/intel_pt/format/tsc:config:10
Note that the default config must be overridden for each term
i.e.
-e intel_pt/noretcomp=0/
is the same as:
-e intel_pt/tsc=1,noretcomp=0/
So, to disable TSC packets use:
-e intel_pt/tsc=0/
It is also possible to specify the config value explicitly:
-e intel_pt/config=0x400/
Note that, as with all events, the event is suffixed with event
modifiers:
u userspace
k kernel
h hypervisor
G guest
H host
p precise ip
h, G and H are for virtualization which is not supported by Intel
PT. p is also not relevant to Intel PT. So only options u and k
are meaningful for Intel PT.
perf_event_attr is displayed if the -vv option is used e.g.
------------------------------------------------------------
perf_event_attr:
type 6
size 112
config 0x400
{ sample_period, sample_freq } 1
sample_type IP|TID|TIME|CPU|IDENTIFIER
read_format ID
disabled 1
inherit 1
exclude_kernel 1
exclude_hv 1
enable_on_exec 1
sample_id_all 1
------------------------------------------------------------
sys_perf_event_open: pid 31104 cpu 0 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 1 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 2 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 3 group_fd -1 flags 0x8
------------------------------------------------------------
config terms
The June 2015 version of Intel 64 and IA-32 Architectures
Software Developer Manuals, Chapter 36 Intel Processor Trace,
defined new Intel PT features. Some of the features are reflect
in new config terms. All the config terms are described below.
tsc Always supported. Produces TSC timestamp packets to provide
timing information. In some cases it is possible to decode
without timing information, for example a per-thread context that
does not overlap executable memory maps.
The default config selects tsc (i.e. tsc=1).
noretcomp Always supported. Disables "return compression" so a
TIP packet is produced when a function returns. Causes more
packets to be produced but might make decoding more reliable.
The default config does not select noretcomp (i.e. noretcomp=0).
psb_period Allows the frequency of PSB packets to be specified.
The PSB packet is a synchronization packet that provides a
starting point for decoding or recovery from errors.
Support for psb_period is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/psb_cyc
which contains "1" if the feature is supported and "0"
otherwise.
Valid values are given by:
/sys/bus/event_source/devices/intel_pt/caps/psb_periods
which contains a hexadecimal value, the bits of which represent
valid values e.g. bit 2 set means value 2 is valid.
The psb_period value is converted to the approximate number of
trace bytes between PSB packets as:
2 ^ (value + 11)
e.g. value 3 means 16KiB bytes between PSBs
If an invalid value is entered, the error message
will give a list of valid values e.g.
$ perf record -e intel_pt/psb_period=15/u uname
Invalid psb_period for intel_pt. Valid values are: 0-5
If MTC packets are selected, the default config selects a value
of 3 (i.e. psb_period=3) or the nearest lower value that is
supported (0 is always supported). Otherwise the default is 0.
If decoding is expected to be reliable and the buffer is large
then a large PSB period can be used.
Because a TSC packet is produced with PSB, the PSB period can
also affect the granularity to timing information in the absence
of MTC or CYC.
mtc Produces MTC timing packets.
MTC packets provide finer grain timestamp information than TSC
packets. MTC packets record time using the hardware crystal
clock (CTC) which is related to TSC packets using a TMA packet.
Support for this feature is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/mtc
which contains "1" if the feature is supported and
"0" otherwise.
The frequency of MTC packets can also be specified - see
mtc_period below.
mtc_period Specifies how frequently MTC packets are produced -
see mtc above for how to determine if MTC packets are supported.
Valid values are given by:
/sys/bus/event_source/devices/intel_pt/caps/mtc_periods
which contains a hexadecimal value, the bits of which represent
valid values e.g. bit 2 set means value 2 is valid.
The mtc_period value is converted to the MTC frequency as:
CTC-frequency / (2 ^ value)
e.g. value 3 means one eighth of CTC-frequency
Where CTC is the hardware crystal clock, the frequency of which
can be related to TSC via values provided in cpuid leaf 0x15.
If an invalid value is entered, the error message
will give a list of valid values e.g.
$ perf record -e intel_pt/mtc_period=15/u uname
Invalid mtc_period for intel_pt. Valid values are: 0,3,6,9
The default value is 3 or the nearest lower value
that is supported (0 is always supported).
cyc Produces CYC timing packets.
CYC packets provide even finer grain timestamp information than
MTC and TSC packets. A CYC packet contains the number of CPU
cycles since the last CYC packet. Unlike MTC and TSC packets,
CYC packets are only sent when another packet is also sent.
Support for this feature is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/psb_cyc
which contains "1" if the feature is supported and
"0" otherwise.
The number of CYC packets produced can be reduced by specifying
a threshold - see cyc_thresh below.
cyc_thresh Specifies how frequently CYC packets are produced -
see cyc above for how to determine if CYC packets are supported.
Valid cyc_thresh values are given by:
/sys/bus/event_source/devices/intel_pt/caps/cycle_thresholds
which contains a hexadecimal value, the bits of which represent
valid values e.g. bit 2 set means value 2 is valid.
The cyc_thresh value represents the minimum number of CPU cycles
that must have passed before a CYC packet can be sent. The
number of CPU cycles is:
2 ^ (value - 1)
e.g. value 4 means 8 CPU cycles must pass before a CYC packet
can be sent. Note a CYC packet is still only sent when another
packet is sent, not at, e.g. every 8 CPU cycles.
If an invalid value is entered, the error message
will give a list of valid values e.g.
$ perf record -e intel_pt/cyc,cyc_thresh=15/u uname
Invalid cyc_thresh for intel_pt. Valid values are: 0-12
CYC packets are not requested by default.
pt Specifies pass-through which enables the branch config term.
The default config selects 'pt' if it is available, so a user will
never need to specify this term.
branch Enable branch tracing. Branch tracing is enabled by
default so to disable branch tracing use branch=0.
The default config selects 'branch' if it is available.
ptw Enable PTWRITE packets which are produced when a ptwrite
instruction is executed.
Support for this feature is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/ptwrite
which contains "1" if the feature is supported and
"0" otherwise.
fup_on_ptw Enable a FUP packet to follow the PTWRITE packet. The
FUP packet provides the address of the ptwrite instruction. In
the absence of fup_on_ptw, the decoder will use the address of
the previous branch if branch tracing is enabled, otherwise the
address will be zero. Note that fup_on_ptw will work even when
branch tracing is disabled.
pwr_evt Enable power events. The power events provide information
about changes to the CPU C-state.
Support for this feature is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/power_event_trace
which contains "1" if the feature is supported and
"0" otherwise.
AUX area sampling option
To select Intel PT "sampling" the AUX area sampling option can be
used:
--aux-sample
Optionally it can be followed by the sample size in bytes e.g.
--aux-sample=8192
In addition, the Intel PT event to sample must be defined e.g.
-e intel_pt//u
Samples on other events will be created containing Intel PT data
e.g. the following will create Intel PT samples on the
branch-misses event, note the events must be grouped using {}:
perf record --aux-sample -e '{intel_pt//u,branch-misses:u}'
An alternative to --aux-sample is to add the config term
aux-sample-size to events. In this case, the grouping is implied
e.g.
perf record -e intel_pt//u -e branch-misses/aux-sample-size=8192/u
is the same as:
perf record -e '{intel_pt//u,branch-misses/aux-sample-size=8192/u}'
but allows for also using an address filter e.g.:
perf record -e intel_pt//u --filter 'filter * @/bin/ls' -e branch-misses/aux-sample-size=8192/u -- ls
It is important to select a sample size that is big enough to
contain at least one PSB packet. If not a warning will be
displayed:
Intel PT sample size (%zu) may be too small for PSB period (%zu)
The calculation used for that is: if sample_size ⟨ psb_period +
256 display the warning. When sampling is used, psb_period
defaults to 0 (2KiB).
The default sample size is 4KiB.
The sample size is passed in aux_sample_size in struct
perf_event_attr. The sample size is limited by the maximum event
size which is 64KiB. It is difficult to know how big the event
might be without the trace sample attached, but the tool
validates that the sample size is not greater than 60KiB.
new snapshot option
The difference between full trace and snapshot from the kernel’s
perspective is that in full trace we don’t overwrite trace data
that the user hasn’t collected yet (and indicated that by
advancing aux_tail), whereas in snapshot mode we let the trace
run and overwrite older data in the buffer so that whenever
something interesting happens, we can stop it and grab a snapshot
of what was going on around that interesting moment.
To select snapshot mode a new option has been added:
-S
Optionally it can be followed by the snapshot size e.g.
-S0x100000
The default snapshot size is the auxtrace mmap size. If neither
auxtrace mmap size nor snapshot size is specified, then the
default is 4MiB for privileged users (or if
/proc/sys/kernel/perf_event_paranoid < 0), 128KiB for
unprivileged users. If an unprivileged user does not specify mmap
pages, the mmap pages will be reduced as described in the new
auxtrace mmap size option section below.
The snapshot size is displayed if the option -vv is used e.g.
Intel PT snapshot size: %zu
new auxtrace mmap size option
Intel PT buffer size is specified by an addition to the -m option
e.g.
-m,16
selects a buffer size of 16 pages i.e. 64KiB.
Note that the existing functionality of -m is unchanged. The
auxtrace mmap size is specified by the optional addition of a
comma and the value.
The default auxtrace mmap size for Intel PT is 4MiB/page_size for
privileged users (or if /proc/sys/kernel/perf_event_paranoid <
0), 128KiB for unprivileged users. If an unprivileged user does
not specify mmap pages, the mmap pages will be reduced from the
default 512KiB/page_size to 256KiB/page_size, otherwise the user
is likely to get an error as they exceed their mlock limit (Max
locked memory as shown in /proc/self/limits). Note that perf does
not count the first 512KiB (actually
/proc/sys/kernel/perf_event_mlock_kb minus 1 page) per cpu
against the mlock limit so an unprivileged user is allowed 512KiB
per cpu plus their mlock limit (which defaults to 64KiB but is
not multiplied by the number of cpus).
In full-trace mode, powers of two are allowed for buffer size,
with a minimum size of 2 pages. In snapshot mode or sampling
mode, it is the same but the minimum size is 1 page.
The mmap size and auxtrace mmap size are displayed if the -vv
option is used e.g.
mmap length 528384
auxtrace mmap length 4198400
Intel PT modes of operation
Intel PT can be used in 3 modes: full-trace mode sample mode
snapshot mode
Full-trace mode traces continuously e.g.
perf record -e intel_pt//u uname
Sample mode attaches a Intel PT sample to other events e.g.
perf record --aux-sample -e intel_pt//u -e branch-misses:u
Snapshot mode captures the available data when a signal is sent
or "snapshot" control command is issued. e.g. using a signal
perf record -v -e intel_pt//u -S ./loopy 1000000000 &
[1] 11435
kill -USR2 11435
Recording AUX area tracing snapshot
Note that the signal sent is SIGUSR2. Note that "Recording AUX
area tracing snapshot" is displayed because the -v option is
used.
The advantage of using "snapshot" control command is that the
access is controlled by access to a FIFO e.g.
$ mkfifo perf.control
$ mkfifo perf.ack
$ cat perf.ack &
[1] 15235
$ sudo ~/bin/perf record --control fifo:perf.control,perf.ack -S -e intel_pt//u -- sleep 60 &
[2] 15243
$ ps -e | grep perf
15244 pts/1 00:00:00 perf
$ kill -USR2 15244
bash: kill: (15244) - Operation not permitted
$ echo snapshot > perf.control
ack
The 3 Intel PT modes of operation cannot be used together.
Buffer handling
There may be buffer limitations (i.e. single ToPa entry) which
means that actual buffer sizes are limited to powers of 2 up to
4MiB (MAX_ORDER). In order to provide other sizes, and in
particular an arbitrarily large size, multiple buffers are
logically concatenated. However an interrupt must be used to
switch between buffers. That has two potential problems: a) the
interrupt may not be handled in time so that the current buffer
becomes full and some trace data is lost. b) the interrupts may
slow the system and affect the performance results.
If trace data is lost, the driver sets truncated in the
PERF_RECORD_AUX event which the tools report as an error.
In full-trace mode, the driver waits for data to be copied out
before allowing the (logical) buffer to wrap-around. If data is
not copied out quickly enough, again truncated is set in the
PERF_RECORD_AUX event. If the driver has to wait, the intel_pt
event gets disabled. Because it is difficult to know when that
happens, perf tools always re-enable the intel_pt event after
copying out data.
Intel PT and build ids
By default "perf record" post-processes the event stream to find
all build ids for executables for all addresses sampled.
Deliberately, Intel PT is not decoded for that purpose (it would
take too long). Instead the build ids for all executables
encountered (due to mmap, comm or task events) are included in
the perf.data file.
To see buildids included in the perf.data file use the command:
perf buildid-list
If the perf.data file contains Intel PT data, that is the same
as:
perf buildid-list --with-hits
Snapshot mode and event disabling
In order to make a snapshot, the intel_pt event is disabled using
an IOCTL, namely PERF_EVENT_IOC_DISABLE. However doing that can
also disable the collection of side-band information. In order to
prevent that, a dummy software event has been introduced that
permits tracking events (like mmaps) to continue to be recorded
while intel_pt is disabled. That is important to ensure there is
complete side-band information to allow the decoding of
subsequent snapshots.
A test has been created for that. To find the test:
perf test list
...
23: Test using a dummy software event to keep tracking
To run the test:
perf test 23
23: Test using a dummy software event to keep tracking : Ok
perf record modes (nothing new here)
perf record essentially operates in one of three modes: per
thread per cpu workload only
"per thread" mode is selected by -t or by --per-thread (with -p
or -u or just a workload). "per cpu" is selected by -C or -a.
"workload only" mode is selected by not using the other options
but providing a command to run (i.e. the workload).
In per-thread mode an exact list of threads is traced. There is
no inheritance. Each thread has its own event buffer.
In per-cpu mode all processes (or processes from the selected
cgroup i.e. -G option, or processes selected with -p or -u) are
traced. Each cpu has its own buffer. Inheritance is allowed.
In workload-only mode, the workload is traced but with per-cpu
buffers. Inheritance is allowed. Note that you can now trace a
workload in per-thread mode by using the --per-thread option.
Privileged vs non-privileged users
Unless /proc/sys/kernel/perf_event_paranoid is set to -1,
unprivileged users have memory limits imposed upon them. That
affects what buffer sizes they can have as outlined above.
The v4.2 kernel introduced support for a context switch metadata
event, PERF_RECORD_SWITCH, which allows unprivileged users to see
when their processes are scheduled out and in, just not by whom,
which is left for the PERF_RECORD_SWITCH_CPU_WIDE, that is only
accessible in system wide context, which in turn requires
CAP_PERFMON or CAP_SYS_ADMIN.
Please see the 45ac1403f564 ("perf: Add PERF_RECORD_SWITCH to
indicate context switches") commit, that introduces these
metadata events for further info.
When working with kernels < v4.2, the following considerations
must be taken, as the sched:sched_switch tracepoints will be used
to receive such information:
Unless /proc/sys/kernel/perf_event_paranoid is set to -1,
unprivileged users are not permitted to use tracepoints which
means there is insufficient side-band information to decode Intel
PT in per-cpu mode, and potentially workload-only mode too if the
workload creates new processes.
Note also, that to use tracepoints, read-access to debugfs is
required. So if debugfs is not mounted or the user does not have
read-access, it will again not be possible to decode Intel PT in
per-cpu mode.
sched_switch tracepoint
The sched_switch tracepoint is used to provide side-band data for
Intel PT decoding in kernels where the PERF_RECORD_SWITCH
metadata event isn’t available.
The sched_switch events are automatically added. e.g. the second
event shown below:
$ perf record -vv -e intel_pt//u uname
------------------------------------------------------------
perf_event_attr:
type 6
size 112
config 0x400
{ sample_period, sample_freq } 1
sample_type IP|TID|TIME|CPU|IDENTIFIER
read_format ID
disabled 1
inherit 1
exclude_kernel 1
exclude_hv 1
enable_on_exec 1
sample_id_all 1
------------------------------------------------------------
sys_perf_event_open: pid 31104 cpu 0 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 1 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 2 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 3 group_fd -1 flags 0x8
------------------------------------------------------------
perf_event_attr:
type 2
size 112
config 0x108
{ sample_period, sample_freq } 1
sample_type IP|TID|TIME|CPU|PERIOD|RAW|IDENTIFIER
read_format ID
inherit 1
sample_id_all 1
exclude_guest 1
------------------------------------------------------------
sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8
sys_perf_event_open: pid -1 cpu 1 group_fd -1 flags 0x8
sys_perf_event_open: pid -1 cpu 2 group_fd -1 flags 0x8
sys_perf_event_open: pid -1 cpu 3 group_fd -1 flags 0x8
------------------------------------------------------------
perf_event_attr:
type 1
size 112
config 0x9
{ sample_period, sample_freq } 1
sample_type IP|TID|TIME|IDENTIFIER
read_format ID
disabled 1
inherit 1
exclude_kernel 1
exclude_hv 1
mmap 1
comm 1
enable_on_exec 1
task 1
sample_id_all 1
mmap2 1
comm_exec 1
------------------------------------------------------------
sys_perf_event_open: pid 31104 cpu 0 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 1 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 2 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 3 group_fd -1 flags 0x8
mmap size 528384B
AUX area mmap length 4194304
perf event ring buffer mmapped per cpu
Synthesizing auxtrace information
Linux
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.042 MB perf.data ]
Note, the sched_switch event is only added if the user is
permitted to use it and only in per-cpu mode.
Note also, the sched_switch event is only added if TSC packets
are requested. That is because, in the absence of timing
information, the sched_switch events cannot be matched against
the Intel PT trace.
By default, perf script will decode trace data found in the
perf.data file. This can be further controlled by new option
--itrace.
New --itrace option
Having no option is the same as
--itrace
which, in turn, is the same as
--itrace=cepwx
The letters are:
i synthesize "instructions" events
b synthesize "branches" events
x synthesize "transactions" events
w synthesize "ptwrite" events
p synthesize "power" events
c synthesize branches events (calls only)
r synthesize branches events (returns only)
e synthesize tracing error events
d create a debug log
g synthesize a call chain (use with i or x)
G synthesize a call chain on existing event records
l synthesize last branch entries (use with i or x)
L synthesize last branch entries on existing event records
s skip initial number of events
q quicker (less detailed) decoding
"Instructions" events look like they were recorded by "perf
record -e instructions".
"Branches" events look like they were recorded by "perf record -e
branches". "c" and "r" can be combined to get calls and returns.
"Transactions" events correspond to the start or end of
transactions. The flags field can be used in perf script to
determine whether the event is a tranasaction start, commit or
abort.
Note that "instructions", "branches" and "transactions" events
depend on code flow packets which can be disabled by using the
config term "branch=0". Refer to the config terms section above.
"ptwrite" events record the payload of the ptwrite instruction
and whether "fup_on_ptw" was used. "ptwrite" events depend on
PTWRITE packets which are recorded only if the "ptw" config term
was used. Refer to the config terms section above. perf script
"synth" field displays "ptwrite" information like this: "ip: 0
payload: 0x123456789abcdef0" where "ip" is 1 if "fup_on_ptw" was
used.
"Power" events correspond to power event packets and CBR
(core-to-bus ratio) packets. While CBR packets are always
recorded when tracing is enabled, power event packets are
recorded only if the "pwr_evt" config term was used. Refer to the
config terms section above. The power events record information
about C-state changes, whereas CBR is indicative of CPU
frequency. perf script "event,synth" fields display information
like this: cbr: cbr: 22 freq: 2189 MHz (200%) mwait: hints: 0x60
extensions: 0x1 pwre: hw: 0 cstate: 2 sub-cstate: 0 exstop: ip: 1
pwrx: deepest cstate: 2 last cstate: 2 wake reason: 0x4 Where:
"cbr" includes the frequency and the percentage of maximum
non-turbo "mwait" shows mwait hints and extensions "pwre" shows
C-state transitions (to a C-state deeper than C0) and whether
initiated by hardware "exstop" indicates execution stopped and
whether the IP was recorded exactly, "pwrx" indicates return to
C0 For more details refer to the Intel 64 and IA-32 Architectures
Software Developer Manuals.
Error events show where the decoder lost the trace. Error events
are quite important. Users must know if what they are seeing is a
complete picture or not. The "e" option may be followed by flags
which affect what errors will or will not be reported. Each flag
must be preceded by either + or -. The flags supported by Intel
PT are: -o Suppress overflow errors -l Suppress trace data lost
errors For example, for errors but not overflow or data lost
errors:
--itrace=e-o-l
The "d" option will cause the creation of a file "intel_pt.log"
containing all decoded packets and instructions. Note that this
option slows down the decoder and that the resulting file may be
very large. The "d" option may be followed by flags which affect
what debug messages will or will not be logged. Each flag must be
preceded by either + or -. The flags support by Intel PT are: -a
Suppress logging of perf events +a Log all perf events By
default, logged perf events are filtered by any specified time
ranges, but flag +a overrides that.
In addition, the period of the "instructions" event can be
specified. e.g.
--itrace=i10us
sets the period to 10us i.e. one instruction sample is
synthesized for each 10 microseconds of trace. Alternatives to
"us" are "ms" (milliseconds), "ns" (nanoseconds), "t" (TSC ticks)
or "i" (instructions).
"ms", "us" and "ns" are converted to TSC ticks.
The timing information included with Intel PT does not give the
time of every instruction. Consequently, for the purpose of
sampling, the decoder estimates the time since the last timing
packet based on 1 tick per instruction. The time on the sample is
not adjusted and reflects the last known value of TSC.
For Intel PT, the default period is 100us.
Setting it to a zero period means "as often as possible".
In the case of Intel PT that is the same as a period of 1 and a
unit of instructions (i.e. --itrace=i1i).
Also the call chain size (default 16, max. 1024) for instructions
or transactions events can be specified. e.g.
--itrace=ig32
--itrace=xg32
Also the number of last branch entries (default 64, max. 1024)
for instructions or transactions events can be specified. e.g.
--itrace=il10
--itrace=xl10
Note that last branch entries are cleared for each sample, so
there is no overlap from one sample to the next.
The G and L options are designed in particular for sample mode,
and work much like g and l but add call chain and branch stack to
the other selected events instead of synthesized events. For
example, to record branch-misses events for ls and then add a
call chain derived from the Intel PT trace:
perf record --aux-sample -e '{intel_pt//u,branch-misses:u}' -- ls
perf report --itrace=Ge
Although in fact G is a default for perf report, so that is the
same as just:
perf report
One caveat with the G and L options is that they work poorly with
"Large PEBS". Large PEBS means PEBS records will be accumulated
by hardware and the written into the event buffer in one go. That
reduces interrupts, but can give very late timestamps. Because
the Intel PT trace is synchronized by timestamps, the PEBS events
do not match the trace. Currently, Large PEBS is used only in
certain circumstances: - hardware supports it - PEBS is used -
event period is specified, instead of frequency - the sample type
is limited to the following flags: PERF_SAMPLE_IP |
PERF_SAMPLE_TID | PERF_SAMPLE_ADDR | PERF_SAMPLE_ID |
PERF_SAMPLE_CPU | PERF_SAMPLE_STREAM_ID | PERF_SAMPLE_DATA_SRC |
PERF_SAMPLE_IDENTIFIER | PERF_SAMPLE_TRANSACTION |
PERF_SAMPLE_PHYS_ADDR | PERF_SAMPLE_REGS_INTR |
PERF_SAMPLE_REGS_USER | PERF_SAMPLE_PERIOD (and sometimes) |
PERF_SAMPLE_TIME Because Intel PT sample mode uses a different
sample type to the list above, Large PEBS is not used with Intel
PT sample mode. To avoid Large PEBS in other cases, avoid
specifying the event period i.e. avoid the perf record -c option,
--count option, or period config term.
To disable trace decoding entirely, use the option --no-itrace.
It is also possible to skip events generated (instructions,
branches, transactions) at the beginning. This is useful to
ignore initialization code.
--itrace=i0nss1000000
skips the first million instructions.
The q option changes the way the trace is decoded. The decoding
is much faster but much less detailed. Specifically, with the q
option, the decoder does not decode TNT packets, and does not
walk object code, but gets the ip from FUP and TIP packets. The q
option can be used with the b and i options but the period is not
used. The q option decodes more quickly, but is useful only if
the control flow of interest is represented or indicated by FUP,
TIP, TIP.PGE, or TIP.PGD packets (refer below). However the q
option could be used to find time ranges that could then be
decoded fully using the --time option.
What will not be decoded with the (single) q option:
• direct calls and jmps
• conditional branches
• non-branch instructions
What will be decoded with the (single) q option:
• asynchronous branches such as interrupts
• indirect branches
• function return target address if the noretcomp config term
(refer config terms section) was used
• start of (control-flow) tracing
• end of (control-flow) tracing, if it is not out of context
• power events, ptwrite, transaction start and abort
• instruction pointer associated with PSB packets
Note the q option does not specify what events will be
synthesized e.g. the p option must be used also to show power
events.
Repeating the q option (double-q i.e. qq) results in even faster
decoding and even less detail. The decoder decodes only extended
PSB (PSB+) packets, getting the instruction pointer if there is a
FUP packet within PSB+ (i.e. between PSB and PSBEND). Note PSB
packets occur regularly in the trace based on the psb_period
config term (refer config terms section). There will be a FUP
packet if the PSB+ occurs while control flow is being traced.
What will not be decoded with the qq option:
• everything except instruction pointer associated with PSB
packets
What will be decoded with the qq option:
• instruction pointer associated with PSB packets
dump option
perf script has an option (-D) to "dump" the events i.e. display
the binary data.
When -D is used, Intel PT packets are displayed. The packet
decoder does not pay attention to PSB packets, but just decodes
the bytes - so the packets seen by the actual decoder may not be
identical in places where the data is corrupt. One example of
that would be when the buffer-switching interrupt has been too
slow, and the buffer has been filled completely. In that case,
the last packet in the buffer might be truncated and immediately
followed by a PSB as the trace continues in the next buffer.
To disable the display of Intel PT packets, combine the -D option
with --no-itrace.
By default, perf report will decode trace data found in the
perf.data file. This can be further controlled by new option
--itrace exactly the same as perf script, with the exception that
the default is --itrace=igxe.
perf inject also accepts the --itrace option in which case
tracing data is removed and replaced with the synthesized events.
e.g.
perf inject --itrace -i perf.data -o perf.data.new
Below is an example of using Intel PT with autofdo. It requires
autofdo (https://github.com/google/autofdo ) and gcc version 5.
The bubble sort example is from the AutoFDO tutorial
(https://gcc.gnu.org/wiki/AutoFDO/Tutorial ) amended to take the
number of elements as a parameter.
$ gcc-5 -O3 sort.c -o sort_optimized
$ ./sort_optimized 30000
Bubble sorting array of 30000 elements
2254 ms
$ cat ~/.perfconfig
[intel-pt]
mispred-all = on
$ perf record -e intel_pt//u ./sort 3000
Bubble sorting array of 3000 elements
58 ms
[ perf record: Woken up 2 times to write data ]
[ perf record: Captured and wrote 3.939 MB perf.data ]
$ perf inject -i perf.data -o inj --itrace=i100usle --strip
$ ./create_gcov --binary=./sort --profile=inj --gcov=sort.gcov -gcov_version=1
$ gcc-5 -O3 -fauto-profile=sort.gcov sort.c -o sort_autofdo
$ ./sort_autofdo 30000
Bubble sorting array of 30000 elements
2155 ms
Note there is currently no advantage to using Intel PT instead of
LBR, but that may change in the future if greater use is made of
the data.
Some hardware has the feature to redirect PEBS records to the
Intel PT trace. Recording is selected by using the aux-output
config term e.g.
perf record -c 10000 -e '{intel_pt/branch=0/,cycles/aux-output/ppp}' uname
Note that currently, software only supports redirecting at most
one PEBS event.
To display PEBS events from the Intel PT trace, use the itrace o
option e.g.
perf script --itrace=oe
For --xed the xed tool is needed. Here is how to install it:
$ git clone https://github.com/intelxed/mbuild.git mbuild
$ git clone https://github.com/intelxed/xed
$ cd xed
$ ./mfile.py --share
$ ./mfile.py examples
$ sudo ./mfile.py --prefix=/usr/local install
$ sudo ldconfig
$ sudo cp obj/examples/xed /usr/local/bin
Basic xed testing:
$ xed | head -3
ERROR: required argument(s) were missing
Copyright (C) 2017, Intel Corporation. All rights reserved.
XED version: [v10.0-328-g7d62c8c49b7b]
$
perf-record(1), perf-script(1), perf-report(1), perf-inject(1)
This page is part of the perf (Performance analysis tools for
Linux (in Linux source tree)) project. Information about the
project can be found at
⟨https://perf.wiki.kernel.org/index.php/Main_Page⟩. If you have a
bug report for this manual page, send it to
linux-kernel@vger.kernel.org. This page was obtained from the
project's upstream Git repository
⟨http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git⟩
on 2020-12-18. (At that time, the date of the most recent commit
that was found in the repository was 2020-12-17.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to man-pages@man7.org
perf 2020-10-14 PERF-INTEL-PT(1)
Pages that refer to this page: perf-inject(1), perf-record(1), perf-report(1), perf-script(1)
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