linux-kernel - Re: [PATCH 10/10] perf record: Add an option to force per-cpu mmaps

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Date:	Fri, 15 Nov 2013 13:27:37 +0200
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Arnaldo Carvalho de Melo <acme@...radead.org>,
	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephane Eranian <eranian@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH 10/10] perf record: Add an option to force per-cpu mmaps

On 15/11/13 13:10, Ingo Molnar wrote:
> 
> * Adrian Hunter <adrian.hunter@...el.com> wrote:
> 
>>> And I also raised why this shouldn't be the default event tracing 
>>> method instead of a weird config option. Per-cpu tracing is cache 
>>> compact, it is easier to size properly and in general it is pretty 
>>> easy to think about. (It also has less of the TSC timestamp 
>>> ordering problems as per thread tracing, at least in theory.)
>>>
>>> Is there something that makes per cpu tracing undesirable as the 
>>> default?
>>
>> One reason is to avoid changing the meaning of existing options.
> 
> Well, the way the tracing buffers are set up is a mostly tool internal 
> matter so in that sense it should be just fine to change the default 
> behavior - as long as output remains unchanged (which it should).
> 
> Or is there any material change in behavior somewhere?

Inheritance is enabled automatically with per-cpu mmaps,
although that is one of the reasons people want
per-cpu mmaps.

> 
>> To flip it around, ignore the patches above and apply:
> 
>> Subject: [PATCH] perf record: Make per-cpu mmaps the default.
> 
> Yay!
> 
>> +--per-thread::
>> +Use per-thread mmaps.  By default per-cpu mmaps are created.  This option
>> +overrides that and uses per-thread mmaps.  A side-effect of that is that
>> +inheritance is automatically disabled.  --per-thread is ignored with a warning
>> +if combined with -a or -C options.
> 
> I think this is the natural thing to do, --per-thread is the 'somewhat 
> weird' option that cannot be used in all modes.
> 
> Acked-by: Ingo Molnar <mingo@...nel.org>
> 
> :-)
> 
> Thanks,
> 
> 	Ingo
> 
> 

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