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

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Date:	Thu, 30 Apr 2009 10:12:54 -0400 (EDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
cc:	Ingo Molnar <mingo@...e.hu>, Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Yuriy Lalym <ylalym@...il.com>, Tejun Heo <tj@...nel.org>,
	ltt-dev@...ts.casi.polymtl.ca,
	Andrew Morton <akpm@...ux-foundation.org>, thomas.pi@...or.dea,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [ltt-dev] [PATCH] Fix dirty page accounting in
 redirty_page_for_writepage()

On Thu, 30 Apr 2009, Mathieu Desnoyers wrote:

> > The 3 variants on x86 generate the same instructions. On other platforms
> > they would need to be able to fallback in various way depending on the
> > availability of instructions that are atomic vs. preempt or irqs.
> >
>
> The problem here, as we did figure out a while ago with the atomic
> slub we worked on a while ago, is that if we have the following code :
>
> local_irq_save
> var++
> var++
> local_irq_restore
>
> that we would like to turn into irq-safe percpu variant with this
> semantic :
>
> percpu_add_irqsafe(var)
> percpu_add_irqsafe(var)
>
> We are generating two irq save/restore in the fallback, which will be
> slow.
>
> However, we could do the following trick :
>
> percpu_irqsave(flags);
> percpu_add_irq(var);
> percpu_add_irq(var);
> percpu_irqrestore(flags);

Hmmm.I do not remember any of those double ops in the patches that I did a
while back for this. It does not make sense either because atomic per cpu
ops are only atomic for a single instruction. You are trying to extend
that so that multiple "atomic" instructions are now atomic.

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