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

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Message-ID: <20131118090117.GE3866@twins.programming.kicks-ass.net>
Date:	Mon, 18 Nov 2013 10:01:17 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	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 Fri, Nov 15, 2013 at 09:41:53AM -0700, David Ahern wrote:
> 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?

I'd say an option is always a possibility, but someone please try what
happens if you use stupid large events (dwarf stack copies) on
PERF_COUNT_SW_PAGE_FAULTS (.period=1) while recording with mmap().

The other option is to simply disallow PERF_SAMPLE_STACK_USER for that
event.

Personally I think 8k copies for every event are way stupid anyway,
that's a metric ton of data at a huge cost.
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