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