linux-kernel - Re: [PATCH] Fix dirty page accounting in redirty_page_for

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Message-ID: <20090430150142.GC20580@elte.hu>
Date:	Thu, 30 Apr 2009 17:01:42 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, thomas.pi@...or.dea,
	Yuriy Lalym <ylalym@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ltt-dev@...ts.casi.polymtl.ca, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] Fix dirty page accounting in
	redirty_page_for_writepage()


* Christoph Lameter <cl@...ux.com> wrote:

> On Thu, 30 Apr 2009, Ingo Molnar wrote:
> 
> > > Yes but sometimes you are already irq safe and such a fallback
> > > would create significant irq/enable/disable stack operations etc
> > > overhead for architectures that are using the fallback.
> >
> > It's a fallback slowpath - non-x86 architectures should still fill
> > in a real implementation of course.
> 
> Arch code cannot provide an effective implementation since they
> always have to assume that interupts need to be disabled if we stay with
> the current implementation.
> 
> > So we first have to see the list of architectures that _cannot_
> > implement an irq-safe op here via a single machine instruction.
> > x86, ia64 and powerpc should be fine.
> 
> Look at Ia64, sparc, s/390, powerpc. They can fall back to atomic 
> ops but those are very ineffective on some of these platforms. 
> Since these are performance critical they will need to be 
> optimized depending on the context of their use in the core.

Could you cite a specific example / situation where you'd use __xxx 
ops?

	Ingo
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