linux-kernel - Re: Perf and ftrace [was Re: PyTimechart]

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Date:	Wed, 12 May 2010 19:11:29 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Pierre Tardy <tardyp@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
	arjan@...radead.org, ziga.mahkovec@...il.com,
	davem <davem@...emloft.net>
Subject: Re: Perf and ftrace [was Re: PyTimechart]


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, 2010-05-12 at 18:46 +0200, Frederic Weisbecker wrote:
> > But there is no strong reason for perf record not to use splice,
> > a part the fact that perf doesn't support splice.
> 
> Its mostly an interface/api question. You cannot easily splice() a mmap()'ed 
> buffer on machines that have address constraints like sparc.
> 
> The thing I was thinking about is adding a new syscall that creates a single 
> buffer of specified size and provides a fd. Then use 
> PERF_EVENT_IOC_SET_OUTPUT, to connect an event to that fd/buffer and use 
> splice() on that fd.
> 
> It could reuse most of the perf buffer code, but simply not map it into 
> userspace and therefore not have the restriction on the vaddr.
> 
> Once you have that, a .splice_read implementation shouldn't be too hard.

That would be a really cool approach. Since most of the performance-sensitive 
streaming happens in the likes of perf record, the actual interface can be 
enhanced with no effect to the end user.

	Ingo
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