Re: [PATCH v5 3/3] Documentation: arm64: Document the PMU event counting threshold feature

[Suzuki K Poulose]

On 20/11/2023 21:31, Namhyung Kim wrote:
> On Mon, Nov 13, 2023 at 3:26 AM James Clark <james.clark@xxxxxxx> wrote:
> > Add documentation for the new Perf event open parameters and
> > the threshold_max capability file.
> >
> > Signed-off-by: James Clark <james.clark@xxxxxxx>
> > ---
> >   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