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

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Message-ID: <20100512175305.GB32496@Krystal>
Date:	Wed, 12 May 2010 13:53:05 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	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>
Subject: Re: Perf and ftrace [was Re: PyTimechart]

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Wed, 2010-05-12 at 13:07 -0400, Mathieu Desnoyers wrote:
> > > Its mostly an interface/api question. You cannot easily splice() a
> > > mmap()'ed buffer on machines that have address constraints like sparc.
> > 
> > Ah ? Can you explain this issue a bit more ? There is possibly a concern I don't
> > quite see here. 
> 
> IIRC Sparc has virtually tagged D-caches on the lower 9 bits of the pfn,
> so to avoid cache aliasing both mappings (kernel and user) need to be
> aligned.

I see.

> 
> Since splice needs to swap pages it needs to allocate replacement pages
> with the exact right alignment, which is rather expensive (in either
> time or space).
> 

Hrm ? Why does splice() _need_ to swap pages exactly ? Or is it purely just the
way the current Ftrace implementation happens to work ? ;)

Would you be fine with an alternative implementation and API that support both
mmap() and splice(), without any need to swap pages ?

Thanks,

Mathieu


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