linux-kernel - 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.

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Message-ID: <87edcflzkr.fsf@linux.intel.com>
Date: Tue, 12 Mar 2024 17:56:04 -0700
From: Andi Kleen <ak@...ux.intel.com>
To: "Wang, Weilin" <weilin.wang@...el.com>
Cc: Namhyung Kim <namhyung@...nel.org>,  Ian Rogers <irogers@...gle.com>,
  Arnaldo Carvalho de Melo <acme@...nel.org>,  Peter Zijlstra
 <peterz@...radead.org>,  Ingo Molnar <mingo@...hat.com>,  Alexander
 Shishkin <alexander.shishkin@...ux.intel.com>,  Jiri Olsa
 <jolsa@...nel.org>,  "Hunter, Adrian" <adrian.hunter@...el.com>,  Kan
 Liang <kan.liang@...ux.intel.com>,  "linux-perf-users@...r.kernel.org"
 <linux-perf-users@...r.kernel.org>,  "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>,  "Taylor, Perry" <perry.taylor@...el.com>,
  "Alt, Samantha" <samantha.alt@...el.com>,  "Biggers, Caleb"
 <caleb.biggers@...el.com>
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.

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

>> -----Original Message-----
>> From: Andi Kleen <ak@...ux.intel.com>
>> Sent: Tuesday, March 12, 2024 5:03 PM
>> To: Wang, Weilin <weilin.wang@...el.com>
>> Cc: Namhyung Kim <namhyung@...nel.org>; Ian Rogers
>> <irogers@...gle.com>; Arnaldo Carvalho de Melo <acme@...nel.org>; Peter
>> Zijlstra <peterz@...radead.org>; Ingo Molnar <mingo@...hat.com>;
>> Alexander Shishkin <alexander.shishkin@...ux.intel.com>; Jiri Olsa
>> <jolsa@...nel.org>; Hunter, Adrian <adrian.hunter@...el.com>; Kan Liang
>> <kan.liang@...ux.intel.com>; linux-perf-users@...r.kernel.org; linux-
>> kernel@...r.kernel.org; Taylor, Perry <perry.taylor@...el.com>; Alt, Samantha
>> <samantha.alt@...el.com>; Biggers, Caleb <caleb.biggers@...el.com>
>> 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@...el.com writes:
>> 
>> > From: Weilin Wang <weilin.wang@...el.com>
>> >
>> > 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