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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Date: Wed, 13 Mar 2024 00:26:26 +0000
From: "Wang, Weilin" <weilin.wang@...el.com>
To: Andi Kleen <ak@...ux.intel.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.



> -----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.

The implementation uses a PIPE to pass the sampled data from perf record instead
of writing the data into a file.

Thanks,
Weilin

> -Andi