Re: [PATCH v2] perf stat: Introduce skippable evsels

[Liang, Kan]

On 2023-04-18 9:00 p.m., Ian Rogers wrote:
> On Tue, Apr 18, 2023 at 5:12 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>>
>> On Tue, Apr 18, 2023 at 2:51 PM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>>>
>>>
>>>
>>> On 2023-04-18 4:08 p.m., Ian Rogers wrote:
>>>> On Tue, Apr 18, 2023 at 11:19 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 2023-04-18 11:43 a.m., Ian Rogers wrote:
>>>>>> On Tue, Apr 18, 2023 at 6:03 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On 2023-04-17 2:13 p.m., Ian Rogers wrote:
>>>>>>>> The json TopdownL1 is enabled if present unconditionally for perf stat
>>>>>>>> default. Enabling it on Skylake has multiplexing as TopdownL1 on
>>>>>>>> Skylake has multiplexing unrelated to this change - at least on the
>>>>>>>> machine I was testing on. We can remove the metric group TopdownL1 on
>>>>>>>> Skylake so that we don't enable it by default, there is still the
>>>>>>>> group TmaL1. To me, disabling TopdownL1 seems less desirable than
>>>>>>>> running with multiplexing - previously to get into topdown analysis
>>>>>>>> there has to be knowledge that "perf stat -M TopdownL1" is the way to
>>>>>>>> do this.
>>>>>>>
>>>>>>> To be honest, I don't think it's a good idea to remove the TopdownL1. We
>>>>>>> cannot remove it just because the new way cannot handle it. The perf
>>>>>>> stat default works well until 6.3-rc7. It's a regression issue of the
>>>>>>> current perf-tools-next.
>>>>>>
>>>>>> I'm not so clear it is a regression to consistently add TopdownL1 for
>>>>>> all architectures supporting it.
>>>>>
>>>>>
>>>>> Breaking the perf stat default is a regression.
>>>>
>>>> Breaking is overstating the use of multiplexing. The impact is less
>>>> accuracy in the IPC and branch misses default metrics,
>>>
>>> Inaccuracy is a breakage for the default.
>>
>> Can you present a case where this matters? The events are already not
>> grouped and so inaccurate for metrics.
> 
> Removing CPUs without perf metrics from the TopdownL1 metric group is
> implemented here:
> https://lore.kernel.org/lkml/20230419005423.343862-6-irogers@xxxxxxxxxx/
> Note, this applies to pre-Icelake and atom CPUs as these also lack
> perf metric (aka topdown) events.
>

That may give the end user the impression that the pre-Icelake doesn't
support the Topdown Level1 events, which is not true.

I think perf should either keep it for all Intel platforms which
supports tma_L1_group, or remove the TopdownL1 name entirely for Intel
platform (let the end user use the tma_L1_group and the name exposed by
the kernel as before.).


> With that change I don't have a case that requires skippable evsels,
> and so we can take that series with patch 6 over the v1 of that series
> with this change.
> 

I'm afraid this is not the only problem the commit 94b1a603fca7 ("perf
stat: Add TopdownL1 metric as a default if present") in the
perf-tools-next branch introduced.

The topdown L2 in the perf stat default on SPR and big core of the ADL
is still missed. I don't see a possible fix for this on the current
perf-tools-next branch.

Thanks,
Kan