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

[Liang, Kan]

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.

> if multiplexing
> is necessary on your Intel architecture. I believe TopdownL1 is more
> useful than either of these metrics and so having TopdownL1 be a
> default is an improvement. We can add a patch, as I keep repeating
> this is off-topic for this patch, to make it so that TopdownL1 isn't
> enabled on Intel CPUs pre-Icelake, but I would discourage this.


We need the TopdownL1. We just don't need TopdownL1 in the perf stat
default when perf metrics feature is not available.


> 
>> The reason we once added the TopdownL1 for ICL and later platform is
>> that it doesn't break the original design (a *clean* output).
> 
> Right, and in 6.3-rc7 the aggregation of counts was broken because of
> duplicated counts and hard coded metrics (I did a last minute partial
> fix). In perf-tools-next aggregation was fixed and we switched to the
> json metrics, that are accurate and up-to-date with the latest TMA
> metrics, so that we wouldn't need to maintain a duplicate code path.
> What keys enabling TopdownL1 in 6.3 is the presence of topdown events
> whilst in perf-tools-next it is the presence of TopdownL1 metric
> group, as this is a more consistent approach and had first been
> proposed by ARM.

A consistent approach is good only when it can benefits all parties
rather than sacrifices any of them.

Apparently, the approach in the perf-tools-next brings all kinds of
issues, multiplexing/inaccuracy in the perf stat default on Intel
platforms. Why cannot we fix it properly before applying the approach?

I think Andi also mentioned the similar request when ARM introduced the
TopdownL1 metrics.
https://lore.kernel.org/linux-perf-users/12e0deef-08db-445f-4958-bcd5c3e10367@xxxxxxxxxxxxxxx/

Thanks,
Kan