linux-kernel - Re: [RFC] Tracer Ring Buffer splice() vs page cache [was: Re: Perf and ftrace [was Re: PyTimechart]]

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Date:	Tue, 18 May 2010 11:16:26 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Pierre Tardy <tardyp@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, arjan@...radead.org,
	ziga.mahkovec@...il.com, davem <davem@...emloft.net>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [RFC] Tracer Ring Buffer splice() vs page cache [was: Re: Perf
	and ftrace [was Re: PyTimechart]]

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Mon, 2010-05-17 at 18:42 -0400, Mathieu Desnoyers wrote:
> > I'll continue to look into this. One of the things I noticed that that we could
> > possibly use the "steal()" operation to steal the pages back from the page cache
> > to repopulate the ring buffer rather than continuously allocating new pages. If
> > steal() fails for some reasons, then we can fall back on page allocation. I'm
> > not sure it is safe to assume anything about pages being in the page cache
> > though. 
> 
> Also, suppose it was still in the page-cache and still dirty, a steal()
> would then punch a hole in the file.

page_cache_pipe_buf_steal starts by doing a wait_on_page_writeback(page); and
then does a try_to_release_page(page, GFP_KERNEL). Only if that succeeds is the
action of stealing succeeding.

> 
> > Maybe the safest route is to just allocate new pages for now.
> 
> Yes, that seems to be the only sane approach.

Yes, a good start anyway.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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