Re: [PATCH v2 3/6] perf pmu: Add support for event.cpus files in sysfs

[Dhananjay Ugwekar]

Hello, Ian, Kan,

On 7/20/2024 3:32 AM, Ian Rogers wrote:
> On Fri, Jul 19, 2024 at 9:35 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>> On 2024-07-19 10:59 a.m., Ian Rogers wrote:
>>> Thanks Kan. I'm not wondering about a case of 2 CPUs, say on CPU0 and
>>> solely its perf event context, I want to know its core power and
>>> package power as a group so I never record one without the other. That
>>> grouping wouldn't be possible with 2 PMUs.
>>
>> For power, to be honest, I don't think it improves anything. It gives
>> users a false image that perf can group these counters.
>> But the truth is that perf cannot. The power counters are all
>> free-running counters. It's impossible to co-schedule them (which
>> requires a global mechanism to disable/enable all counters, e.g.,
>> GLOBAL_CTRL for core PMU). The kernel still has to read the counters one
>> by one while the counters keep running. There are no differences with or
>> without a group for the power events.
> 
> Ok, so power should copy cstate with _core, _pkg, etc. I agree the
> difference is small and I like the idea of being consistent.

So, it seems we want to follow the new PMU addition approach for RAPL 
being consistent with Intel cstate driver, should I revive my "power_per_core" 
PMU thread now?

Thanks,
Dhananjay

 Do we
> want to add "event.cpus" support to the tool anyway for potential
> future uses? This would at least avoid problems with newer kernels and
> older perf tools were we to find a good use for it.
> 
>>> My understanding had been that for core PMUs a "perf stat -C" option
>>> would choose the particular CPU to count the event on, for an uncore
>>> PMU the -C option would override the cpumask's "default" value. We
>>> have code to validate this:
>>> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/evlist.c?h=perf-tools-next#n2522
>>> But it seems now that overriding an uncore PMU's default CPU is
>>> ignored.
>>
>> For the uncore driver, no matter what -C set, it writes the default CPU
>> back.
>> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/arch/x86/events/intel/uncore.c#n760
>>
>>> If you did:
>>> $ perf stat -C 1 -e data_read -a sleep 0.1
>>> Then the tool thinks data_read is on CPU1 and will set its thread
>>> affinity to CPU1 to avoid IPIs. It seems to fix this we need to just
>>> throw away the -C option.
>> The perf tool can still read the the counter from CPU1 and no IPIs
>> because of the PMU_EV_CAP_READ_ACTIVE_PKG().
>>
>> Not quite sure, but it seems only the open and close may be impacted and
>> silently changed to CPU0.
> 
> There's also enable/disable. Andi did the work and there were some
> notable gains but likely more on core events. Ultimately it'd be nice
> to be opening, closing.. everything in parallel given the calls are
> slow and the work is embarrassingly parallel.
> It feels like the cpumasks for uncore could still do with some cleanup
> wrt -C I'm just unsure at the moment what this should be. Tbh, I'm
> tempted to rewrite evlist propagate maps as someone may look at it and
> think I believe in what it is doing. The parallel stuff we should grab
> Riccardo's past work.
> 
> Thanks,
> Ian
> 
> 
>> Thanks,
>> Kan
>>>
>>>>> 2) do the /sys/devices/<pmu>/events/event.(unit|scale|per-pkg|snapshot)
>>>>> files parse correctly and have a corresponding event.
>>>>> 3) keep adding opening events on the PMU to a group to make sure that
>>>>> when counters are exhausted the perf_event_open fails (I've seen this
>>>>> bug on AMD)
>>>>> 4) are the values in the type file unique
>>>>>
>>>>
>>>> The rest sounds good to me.
>>>
>>> Cool. Let me know if you can think of more.
>>>
>>> Thanks,
>>> Ian
>>>
>>>> Thanks,
>>>> Kan
>>>