+++ /dev/null
-Intel Processor Trace
-=====================
-
-Overview
-========
-
-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.
-
-
-Quickstart
-==========
-
-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. A script perf-with-kcore can do that, but beware that the
-script makes use of 'sudo' to copy /proc/kcore. If you have perf installed
-locally from the source tree you can do:
-
- ~/libexec/perf-core/perf-with-kcore record pt_ls -e intel_pt// -- ls
-
-which will create a directory named 'pt_ls' and put the perf.data file and
-copies of /proc/kcore, /proc/kallsyms and /proc/modules into it. Then to use
-'perf report' becomes:
-
- ~/libexec/perf-core/perf-with-kcore report pt_ls
-
-Because samples are synthesized after-the-fact, the sampling period can be
-selected for reporting. e.g. sample every microsecond
-
- ~/libexec/perf-core/perf-with-kcore 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.
-
-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.
-
-
-perf record
-===========
-
-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 2 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 e.g.
-
- 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 2 modes 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_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.
-
-
-perf script
-===========
-
-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)
- l synthesize last branch entries (use with i or x)
- s skip initial number of events
-
-"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 "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.
-
-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.
-
-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.
-
-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.
-
-
-perf report
-===========
-
-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
-===========
-
-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.
-
-
-PEBS via Intel PT
-=================
-
-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
--- /dev/null
+perf-intel-pt(1)
+================
+
+NAME
+----
+perf-intel-pt - Support for Intel Processor Trace within perf tools
+
+SYNOPSIS
+--------
+[verse]
+'perf record' -e intel_pt//
+
+DESCRIPTION
+-----------
+
+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.
+
+
+Quickstart
+----------
+
+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. A script perf-with-kcore can do that, but beware that the
+script makes use of 'sudo' to copy /proc/kcore. If you have perf installed
+locally from the source tree you can do:
+
+ ~/libexec/perf-core/perf-with-kcore record pt_ls -e intel_pt// -- ls
+
+which will create a directory named 'pt_ls' and put the perf.data file and
+copies of /proc/kcore, /proc/kallsyms and /proc/modules into it. Then to use
+'perf report' becomes:
+
+ ~/libexec/perf-core/perf-with-kcore report pt_ls
+
+Because samples are synthesized after-the-fact, the sampling period can be
+selected for reporting. e.g. sample every microsecond
+
+ ~/libexec/perf-core/perf-with-kcore 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.
+
+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.
+
+
+perf record
+-----------
+
+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 2 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 e.g.
+
+ 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 2 modes 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_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.
+
+
+perf script
+-----------
+
+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)
+ l synthesize last branch entries (use with i or x)
+ s skip initial number of events
+
+"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 "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.
+
+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.
+
+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.
+
+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.
+
+
+perf report
+-----------
+
+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
+-----------
+
+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.
+
+
+PEBS via Intel PT
+-----------------
+
+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
projects / openwrt / staging / blogic.git / commitdiff