Messages in this thread | | | Date | Tue, 21 Nov 2023 10:33:38 +0000 | Subject | Re: [PATCH v5 3/3] Documentation: arm64: Document the PMU event counting threshold feature | From | Suzuki K Poulose <> |
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On 20/11/2023 21:31, Namhyung Kim wrote: > On Mon, Nov 13, 2023 at 3:26 AM James Clark <james.clark@arm.com> wrote: >> >> Add documentation for the new Perf event open parameters and >> the threshold_max capability file. >> >> Signed-off-by: James Clark <james.clark@arm.com> >> --- >> Documentation/arch/arm64/perf.rst | 56 +++++++++++++++++++++++++++++++ >> 1 file changed, 56 insertions(+) >> >> diff --git a/Documentation/arch/arm64/perf.rst b/Documentation/arch/arm64/perf.rst >> index 1f87b57c2332..36b8111a710d 100644 >> --- a/Documentation/arch/arm64/perf.rst >> +++ b/Documentation/arch/arm64/perf.rst >> @@ -164,3 +164,59 @@ and should be used to mask the upper bits as needed. >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/arch/arm64/tests/user-events.c >> .. _tools/lib/perf/tests/test-evsel.c: >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/perf/tests/test-evsel.c >> + >> +Event Counting Threshold >> +========================================== >> + >> +Overview >> +-------- >> + >> +FEAT_PMUv3_TH (Armv8.8) permits a PMU counter to increment only on >> +events whose count meets a specified threshold condition. For example if >> +threshold_compare is set to 2 ('Greater than or equal'), and the >> +threshold is set to 2, then the PMU counter will now only increment by >> +when an event would have previously incremented the PMU counter by 2 or >> +more on a single processor cycle. >> + >> +To increment by 1 after passing the threshold condition instead of the >> +number of events on that cycle, add the 'threshold_count' option to the >> +commandline. >> + >> +How-to >> +------ >> + >> +The threshold, threshold_compare and threshold_count values can be >> +provided per event: >> + >> +.. code-block:: sh >> + >> + perf stat -e stall_slot/threshold=2,threshold_compare=2/ \ >> + -e dtlb_walk/threshold=10,threshold_compare=3,threshold_count/ > > Can you please explain this a bit more? > > I guess the first event counts stall_slot PMU if the event if it's > greater than or equal to 2. And as threshold_count is not set, > it'd count the stall_slot as is. E.g. it counts 3 when it sees 3. > > OTOH, dtlb_walk will count 1 if it sees an event less than 10. > Is my understanding correct?
That is correct. The behavior is described in the paragraph above. But I agree that it would be really helpful if we explained with the example above.
> >> + >> +And the following comparison values are supported: >> + >> +.. code-block:: >> + >> + 0: Not-equal >> + 1: Equals >> + 2: Greater-than-or-equal >> + 3: Less-than > > So the above values are for threashold_compare, right? > It'd be nice if it's more explicit. > > Similarly, it'd be helpful to have a description for the > threshold and threshold_count fields.
Agreed.
Suzuki
> > Thanks, > Namhyung > >> + >> +The maximum supported threshold value can be read from the caps of each >> +PMU, for example: >> + >> +.. code-block:: sh >> + >> + cat /sys/bus/event_source/devices/armv8_pmuv3/caps/threshold_max >> + >> + 0x000000ff >> + >> +If a value higher than this is given, then it will be silently clamped >> +to the maximum. The highest possible maximum is 4095, as the config >> +field for threshold is limited to 12 bits, and the Perf tool will refuse >> +to parse higher values. >> + >> +If the PMU doesn't support FEAT_PMUv3_TH, then threshold_max will read >> +0, and both threshold and threshold_compare will be silently ignored. >> +threshold_max will also read as 0 on aarch32 guests, even if the host >> +is running on hardware with the feature. >> -- >> 2.34.1 >> >>
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