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

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Date:	Fri, 15 Nov 2013 09:41:53 -0700
From:	David Ahern <dsahern@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.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, 4:34 AM, Peter Zijlstra 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.
>
> You can always screw yourself over using this stuff, no exceptions.
>

What now? Can we add the mmap path as an option? Leave write by default 
and users can select mmap if they want? e.g., have out_size default to 0 
and 'perf trace record' can set --out-pages to 64M. It bypasses all 
system calls at the expense of more page faults.

David
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