linux-kernel - Re: [PATCH V3 06/11] perf record: Add an option to force per-cpu mmaps

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Date:	Tue, 12 Nov 2013 18:48:25 -0800
From:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Jiri Olsa <jolsa@...hat.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH V3 06/11] perf record: Add an option to force per-cpu
 mmaps

Ingo Molnar [mingo@...nel.org] wrote:
| 
| * Peter Zijlstra <peterz@...radead.org> wrote:
| 
| > On Tue, Nov 05, 2013 at 02:31:52PM -0300, Arnaldo Carvalho de Melo wrote:
| > > PeterZ,
| > > 
| > > 	Can I have your Acked-by for this one? I guess now the goal is
| > > achieved, no?

Being able to profile children with the --pid is a big plus.

| > 
| > So this option allows -t/-p/-u to create one buffer per cpu and attach
| > all the various thread/process/user tasks' their counters to that one
| > buffer?
| > 
| > As opposed to the current state where each such counter would have its
| > own buffer.
| > 
| > If this is what the patch does, then yes, although I would prefer a
| > slightly clearer Changelog.
| > 
| > Acked-by: Peter Zijlstra <peterz@...radead.org>
| 
| Is there any reason why we wouldn't want to make this the default 
| behavior?
| 
| That way we could also lose the somewhat suboptimal 'force' naming: 
| there's nothing forced really, we simply switch to another ring-buffer 
| setup ...

It would be also good if the man page added a comment on when a user
would want one ring-buffer setup over the other.

If the main benefit is to have the children profiled when tasks are 
specified, how about changing the option to --inherit (-I) ?

Or for consistency with 'perf record <application>', have --pid profile 
children by default and let users specify --no-inherit with --pid if 
they don't want children profiled.

Sukadev

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