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

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Message-ID: <20131113113439.GI21461@twins.programming.kicks-ass.net>
Date:	Wed, 13 Nov 2013 12:34:39 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	David Ahern <dsahern@...il.com>,
	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 Tue, Nov 12, 2013 at 10:11:21PM +0100, Ingo Molnar wrote:
> 
> * David Ahern <dsahern@...il.com> wrote:
> 
> > > Dunno.. it _should_ all work. Try it and see what it does.. Once the 
> > > events are bigger than a page things might get 'interesting' though.
> 
> Which could be the case with call-graph recording, right?

Not typically, I think we're limiting call graphs to 127 u64, which is
~1k. Maybe you can blow the single page if you also do a large
top-of-stack copy for dwarf/unwind nonsense.

> > 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.
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