linux-kernel - Re: [PATCH 2/9] perf: Deny optimized switch for events read by PERF_SAMPLE

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Date:	Mon, 8 Sep 2014 12:11:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Jiri Olsa <jolsa@...nel.org>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Jen-Cheng(Tommy) Huang" <tommy24@...ech.edu>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 2/9] perf: Deny optimized switch for events read by
 PERF_SAMPLE_READ

On Mon, Sep 08, 2014 at 12:00:19PM +0200, Jiri Olsa wrote:

> > no.. so the value of the counter is the sum of all the inherited events.
> > It doesn't matter if you flip it or not the sum is not affected.
> > 
> > PERF_SAMPLE_READ should return the value.
> 
> so I want to be able to do the leader sampling over child processes
> that means:
>   - have event group with sampling leader, and the rest
>     of the group events' periods being read on leader's sample
>     via PERF_SAMPLE_READ sample_type
>   - for each child process created I want it to do the
>     same thing as the parent - sample on leader, read
>     values of other events in group via PERF_SAMPLE_READ

The thing is, PERF_SAMPLE_READ should return the exact same thing as an
actual read(), and for read() the value reported is the sum of all
events.

I _think_ I know what you want, and I think I see why you'd want that,
but I'm not sure how to best do that.. Hmm.. maybe look at
'perf_event_attr::inherit_stat' that appears to be similar.

At which point read() vs inherit_stat might be 'broken'. Hard to know if
anybody uses that :-(

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