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

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Date:   Thu, 23 Nov 2023 15:33:16 +0000
From:   James Clark <james.clark@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>,
        Namhyung Kim <namhyung@...il.com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, will@...nel.org,
        mark.rutland@....com, Catalin Marinas <catalin.marinas@....com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/3] Documentation: arm64: Document the PMU event
 counting threshold feature



On 21/11/2023 10:33, Suzuki K Poulose wrote:
> On 20/11/2023 21:31, Namhyung Kim wrote:
>> On Mon, Nov 13, 2023 at 3:26 AM James Clark <james.clark@....com> wrote:
>>>
>>> Add documentation for the new Perf event open parameters and
>>> the threshold_max capability file.
>>>
>>> Signed-off-by: James Clark <james.clark@....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.
> 

Yeah I can add a description of how the example behaves.

>>
>>> +
>>> +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.

Yep I agree, I can label this with threshold_compare.

>>
>> Similarly, it'd be helpful to have a description for the
>> threshold and threshold_count fields.
> 
> Agreed.
> 
> Suzuki
> 

Yeah I'll add explicit descriptions for each field.

Thanks for the review.

> 
> 
>>
>> 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
>>>
>>>
> 
>