Re: [RFC PATCH v4 2/6] perf stat: Fork and launch perf record when perf stat needs to get retire latency value for a metric.

[Andi Kleen]

"Wang, Weilin" <weilin.wang@xxxxxxxxx> writes:

>> -----Original Message-----
>> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>> Sent: Tuesday, March 12, 2024 5:03 PM
>> To: Wang, Weilin <weilin.wang@xxxxxxxxx>
>> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>; Ian Rogers
>> <irogers@xxxxxxxxxx>; Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>; Peter
>> Zijlstra <peterz@xxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>;
>> Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>; Jiri Olsa
>> <jolsa@xxxxxxxxxx>; Hunter, Adrian <adrian.hunter@xxxxxxxxx>; Kan Liang
>> <kan.liang@xxxxxxxxxxxxxxx>; linux-perf-users@xxxxxxxxxxxxxxx; linux-
>> kernel@xxxxxxxxxxxxxxx; Taylor, Perry <perry.taylor@xxxxxxxxx>; Alt, Samantha
>> <samantha.alt@xxxxxxxxx>; Biggers, Caleb <caleb.biggers@xxxxxxxxx>
>> Subject: Re: [RFC PATCH v4 2/6] perf stat: Fork and launch perf record when
>> perf stat needs to get retire latency value for a metric.
>> 
>> weilin.wang@xxxxxxxxx writes:
>> 
>> > From: Weilin Wang <weilin.wang@xxxxxxxxx>
>> >
>> > When retire_latency value is used in a metric formula, perf stat would fork a
>> > perf record process with "-e" and "-W" options. Perf record will collect
>> > required retire_latency values in parallel while perf stat is collecting
>> > counting values.
>> 
>> How does that work when the workload is specified on the command line?
>> The workload would run twice? That is very inefficient and may not
>> work if it's a large workload.
>> 
>> The perf tool infrastructure is imho not up to the task of such
>> parallel collection.
>> 
>> Also it won't work for very long collections because you will get a
>> very large perf.data. Better to use a pipeline.
>> 
>> I think it would be better if you made it a separate operation that can
>> generate a file that is then consumed by perf stat. This is also more efficient
>> because often the calibration is only needed once. And it's all under
>> user control so no nasty surprises.
>> 
>
> Workload runs only once with perf stat. Perf record is forked by perf stat and run
> in parallel with perf stat. Perf stat will send perf record a signal to terminate after 
> perf stat stops collecting count value.

I don't understand how the perf record filters on the workload created by
the perf stat. At a minimum you would need -p to connect to the pid
of the parent, but IIRC -p doesnt follow children, so if it forked
it wouldn't work.

I think your approach may only work with -a, but perhaps I'm missing
something (-a is often not usable due to restrictions)

Also if perf stat runs in interval mode and you only get the data
at the end how would that work?

iirc i wrestled with all these questions for toplev (which has a
similar feature) and in the end i concluded doing it automatically
has far too many problems.

-Andi