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

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Date:	Wed, 12 May 2010 19:47:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
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]

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.

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).


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