Re: [PATCH] perf: docs: Add man page entry for Arm SPE

[Leo Yan]

On Wed, Apr 13, 2022 at 09:40:21AM +0100, James Clark wrote:
> The SPE integration in Perf has quite a few usability quirks that
> can't be found by just reading the reference manual. So document this
> and at the same time add a summary of the feature that is also hard to
> find elsewhere.
> 
> Co-authored-by: Luke Dare <Luke.Dare@xxxxxxx>
> Co-authored-by: Al Grant <al.grant@xxxxxxx>
> Signed-off-by: James Clark <james.clark@xxxxxxx>

Reviewed-by: Leo Yan <leo.yan@xxxxxxxxxx>

> ---
>  tools/perf/Documentation/perf-arm-spe.txt | 218 ++++++++++++++++++++++
>  tools/perf/Documentation/perf.txt         |   2 +-
>  2 files changed, 219 insertions(+), 1 deletion(-)
>  create mode 100644 tools/perf/Documentation/perf-arm-spe.txt
> 
> diff --git a/tools/perf/Documentation/perf-arm-spe.txt b/tools/perf/Documentation/perf-arm-spe.txt
> new file mode 100644
> index 000000000000..bf03222e9a68
> --- /dev/null
> +++ b/tools/perf/Documentation/perf-arm-spe.txt
> @@ -0,0 +1,218 @@
> +perf-arm-spe(1)
> +================
> +
> +NAME
> +----
> +perf-arm-spe - Support for Arm Statistical Profiling Extension within Perf tools
> +
> +SYNOPSIS
> +--------
> +[verse]
> +'perf record' -e arm_spe//
> +
> +DESCRIPTION
> +-----------
> +
> +The SPE (Statistical Profiling Extension) feature provides accurate attribution of latencies and
> + events down to individual instructions. Rather than being interrupt-driven, it picks an
> +instruction to sample and then captures data for it during execution. Data includes execution time
> +in cycles. For loads and stores it also includes data address, cache miss events, and data origin.
> +
> +The sampling has 5 stages:
> +
> +  1. Choose an operation
> +  2. Collect data about the operation
> +  3. Optionally discard the record based on a filter
> +  4. Write the record to memory
> +  5. Interrupt when the buffer is full
> +
> +Choose an operation
> +~~~~~~~~~~~~~~~~~~~
> +
> +This is chosen from a sample population, for SPE this is an IMPLEMENTATION DEFINED choice of all
> +architectural instructions or all micro-ops. Sampling happens at a programmable interval. The
> +architecture provides a mechanism for the SPE driver to infer the minimum interval at which it should
> +sample. This minimum interval is used by the driver if no interval is specified. A pseudo-random
> +perturbation is also added to the sampling interval by default.
> +
> +Collect data about the operation
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +Program counter, PMU events, timings and data addresses related to the operation are recorded.
> +Sampling ensures there is only one sampled operation is in flight.
> +
> +Optionally discard the record based on a filter
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +Based on programmable criteria, choose whether to keep the record or discard it. If the record is
> +discarded then the flow stops here for this sample.
> +
> +Write the record to memory
> +~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +The record is appended to a memory buffer
> +
> +Interrupt when the buffer is full
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +When the buffer fills, an interrupt is sent and the driver signals Perf to collect the records.
> +Perf saves the raw data in the perf.data file.
> +
> +Opening the file
> +----------------
> +
> +Up until this point no decoding of the SPE data was done by either the kernel or Perf. Only when the
> +recorded file is opened with 'perf report' or 'perf script' does the decoding happen. When decoding
> +the data, Perf generates "synthetic samples" as if these were generated at the time of the
> +recording. These samples are the same as if normal sampling was done by Perf without using SPE,
> +although they may have more attributes associated with them. For example a normal sample may have
> +just the instruction pointer, but an SPE sample can have data addresses and latency attributes.
> +
> +Why Sampling?
> +-------------
> +
> + - Sampling, rather than tracing, cuts down the profiling problem to something more manageable for
> + hardware. Only one sampled operation is in flight at a time.
> +
> + - Allows precise attribution data, including: Full PC of instruction, data virtual and physical
> + addresses.
> +
> + - Allows correlation between an instruction and events, such as TLB and cache miss. (Data source
> + indicates which particular cache was hit, but the meaning is implementation defined because
> + different implementations can have different cache configurations.)
> +
> +However, SPE does not provide any call-graph information, and relies on statistical methods.
> +
> +Collisions
> +----------
> +
> +When an operation is sampled while a previous sampled operation has not finished, a collision
> +occurs. The new sample is dropped. Collisions affect the integrity of the data, so the sample rate
> +should be set to avoid collisions.
> +
> +The 'sample_collision' PMU event can be used to determine the number of lost samples. Although this
> +count is based on collisions _before_ filtering occurs. Therefore this can not be used as an exact
> +number for samples dropped that would have made it through the filter, but can be a rough
> +guide.
> +
> +The effect of microarchitectural sampling
> +-----------------------------------------
> +
> +If an implementation samples micro-operations instead of instructions, the results of sampling must
> +be weighted accordingly.
> +
> +For example, if a given instruction A is always converted into two micro-operations, A0 and A1, it
> +becomes twice as likely to appear in the sample population.
> +
> +The coarse effect of conversions, and, if applicable, sampling of speculative operations, can be
> +estimated from the 'sample_pop' and 'inst_retired' PMU events.
> +
> +Kernel Requirements
> +-------------------
> +
> +The ARM_SPE_PMU config must be set to build as either a module or statically.
> +
> +Depending on CPU model, the kernel may need to be booted with page table isolation disabled
> +(kpti=off). If KPTI needs to be disabled, this will fail with a console message "profiling buffer
> +inaccessible. Try passing 'kpti=off' on the kernel command line".
> +
> +Capturing SPE with perf command-line tools
> +------------------------------------------
> +
> +You can record a session with SPE samples:
> +
> +  perf record -e arm_spe// -- ./mybench
> +
> +The sample period is set from the -c option, and because the minimum interval is used by default
> +it's recommended to set this to a higher value. The value is written to PMSIRR.INTERVAL.
> +
> +Config parameters
> +~~~~~~~~~~~~~~~~~
> +
> +These are placed between the // in the event and comma separated. For example '-e
> +arm_spe/load_filter=1,min_latency=10/'
> +
> +  branch_filter=1     - collect branches only (PMSFCR.B)
> +  event_filter=<mask> - filter on specific events (PMSEVFR) - see bitfield description below
> +  jitter=1            - use jitter to avoid resonance when sampling (PMSIRR.RND)
> +  load_filter=1       - collect loads only (PMSFCR.LD)
> +  min_latency=<n>     - collect only samples with this latency or higher* (PMSLATFR)
> +  pa_enable=1         - collect physical address (as well as VA) of loads/stores (PMSCR.PA) - requires privilege
> +  pct_enable=1        - collect physical timestamp instead of virtual timestamp (PMSCR.PCT) - requires privilege
> +  store_filter=1      - collect stores only (PMSFCR.ST)
> +  ts_enable=1         - enable timestamping with value of generic timer (PMSCR.TS)
> +
> ++++*+++ Latency is the total latency from the point at which sampling started on that instruction, rather
> +than only the execution latency.
> +
> +Only some events can be filtered on; these include:
> +
> +  bit 1     - instruction retired (i.e. omit speculative instructions)
> +  bit 3     - L1D refill
> +  bit 5     - TLB refill
> +  bit 7     - mispredict
> +  bit 11    - misaligned access
> +
> +So to sample just retired instructions:
> +
> +  perf record -e arm_spe/event_filter=2/ -- ./mybench
> +
> +or just mispredicted branches:
> +
> +  perf record -e arm_spe/event_filter=0x80/ -- ./mybench
> +
> +Viewing the data
> +~~~~~~~~~~~~~~~~~
> +
> +By default perf report and perf script will assign samples to separate groups depending on the
> +attributes/events of the SPE record. Because instructions can have multiple events associated with
> +them, the samples in these groups are not necessarily unique. For example perf report shows these
> +groups:
> +
> +  Available samples
> +  0 arm_spe//
> +  0 dummy:u
> +  21 l1d-miss
> +  897 l1d-access
> +  5 llc-miss
> +  7 llc-access
> +  2 tlb-miss
> +  1K tlb-access
> +  36 branch-miss
> +  0 remote-access
> +  900 memory
> +
> +The arm_spe// and dummy:u events are implementation details and are expected to be empty.
> +
> +To get a full list of unique samples that are not sorted into groups, set the itrace option to
> +generate 'instruction' samples. The period option is also taken into account, so set it to 1
> +instruction unless you want to further downsample the already sampled SPE data:
> +
> +  perf report --itrace=i1i
> +
> +Memory access details are also stored on the samples and this can be viewed with:
> +
> +  perf report --mem-mode
> +
> +Common errors
> +~~~~~~~~~~~~~
> +
> + - "Cannot find PMU `arm_spe'. Missing kernel support?"
> +
> +   Module not built or loaded, KPTI not disabled (see above), or running on a VM
> +
> + - "Arm SPE CONTEXT packets not found in the traces."
> +
> +   Root privilege is required to collect context packets. But these only increase the accuracy of
> +   assigning PIDs to kernel samples. For userspace sampling this can be ignored.
> +
> + - Excessively large perf.data file size
> +
> +   Increase sampling interval (see above)
> +
> +
> +SEE ALSO
> +--------
> +
> +linkperf:perf-record[1], linkperf:perf-script[1], linkperf:perf-report[1],
> +linkperf:perf-inject[1]
> diff --git a/tools/perf/Documentation/perf.txt b/tools/perf/Documentation/perf.txt
> index 71ebdf8125de..ba3df49c169d 100644
> --- a/tools/perf/Documentation/perf.txt
> +++ b/tools/perf/Documentation/perf.txt
> @@ -77,7 +77,7 @@ linkperf:perf-stat[1], linkperf:perf-top[1],
>  linkperf:perf-record[1], linkperf:perf-report[1],
>  linkperf:perf-list[1]
>  
> -linkperf:perf-annotate[1],linkperf:perf-archive[1],
> +linkperf:perf-annotate[1],linkperf:perf-archive[1],linkperf:perf-arm-spe[1],
>  linkperf:perf-bench[1], linkperf:perf-buildid-cache[1],
>  linkperf:perf-buildid-list[1], linkperf:perf-c2c[1],
>  linkperf:perf-config[1], linkperf:perf-data[1], linkperf:perf-diff[1],
> -- 
> 2.28.0
>