linux-kernel - Re: [PATCH 4/5] perf record: mmap output file

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Date:	Wed, 13 Nov 2013 07:29:22 -0700
From:	David Ahern <dsahern@...il.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org, jolsa@...hat.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 4/5] perf record: mmap output file - v5

On 11/13/13, 4:50 AM, Ingo Molnar wrote:
>>>> one option here is not allow page faults and system wide system
>>>> calls. system wide tracing needs mmap; page faults for a task can
>>>> use write(). I left that option in case something like this came up.
>>>
>>> So maybe splice() sounds like the right long term solution after all?
>>> :-/
>>
>> Right until you put a tracepoint (kprobe) somewhere in whatever function
>> is used to transfer a single page into/from a splice pipe.
>
> That ought to be a far less common occurance than tracing page faults
> though.
>
>> You can always screw yourself over using this stuff, no exceptions.
>
> Granted, as the many notrace markings demonstrate this stuff really wants
> to observe itself observing itself all the time! :)

perf can have options for output -- write(), mmap(), splice() -- and 
some logic of when not to use it:

1. don't use write for system wide tracing

2. don't use mmap for page fault tracing

3. don't use splice for .... (need a splice implementation)

and allow the user to override.

David
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