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: <ZfI-JqTZevIhEa3o@tassilo>
Date: Wed, 13 Mar 2024 17:00:38 -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.

> We could use -a -C and cgroup together. I think this could be a useful use case.
> There could be other improvement to the implementation in next step. But I believe

I don't know how you would improve it. A lot of the problems are fairly
fundamental.

> current implementation could provide users the access to our new feature with 
> accurate results 
> and without adding too much overhead. 

perf record increases the overhead a lot over a perf stat! Sampling
is much more expensive than counting.

That should be at least a order of magnitude difference.
Another advantage of doing it separately.

That will also make it inaccurate.

Please do a proper implementation. This one is no good.

You can likely reuse a lot of your code:

- Add a perf calibrate to run the profile step separately that generates the JSON
- Add a --metrics option to perf to read the resulting JSON
- Add some mechanism to read a default JSON as fallback. I guess this
one could be compiled in to avoid a dependency on an installed file.

-Andi