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

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Date:	Fri, 1 May 2009 23:00:21 -0400 (EDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [ltt-dev] [PATCH] Fix dirty page accounting in
 redirty_page_for_writepage()

On Fri, 1 May 2009, Mathieu Desnoyers wrote:

> Then, if I understand you correctly, you propose :
>
> void __inc_zone_state(struct zone *zone, enum zone_stat_item item)
> {
>         ... assuming p references percpu "u8" counters ...
>         u8 p_new;
>
>         p_new = percpu_add_return(p, 1);
>
>         if (unlikely(!(p_new & pcp->stat_threshold_mask)))
>                 zone_page_state_add(pcp->stat_threshold, zone, item);
> }
>
> void inc_zone_state(struct zone *zone, enum zone_stat_item item)
> {
>         ... assuming p references percpu "u8" counters ...
>         u8 p_new;
>
>         p_new = percpu_add_return_irqsafe(p, 1);
>
>         if (unlikely(!(p_new & pcp->stat_threshold_mask)))
>                 zone_page_state_add(pcp->stat_threshold, zone, item);
> }
>
> (therefore opting for code duplication)
>
> Am I correct ?

Well __inc_zone_state is fine by itself. inc_zone_state will currently
disable irqs. But we can do it your way and duplicate the code.

> void __inc_zone_state(struct zone *zone, enum zone_stat_item item)
> {
>         ... assuming p references percpu "u8" counters ...
>         u8 p_new;
>
>         p_new = __percpu_add_return_irqsafe(p, 1);
>
>         if (unlikely(!(p_new & pcp->stat_threshold_mask)))
>                 zone_page_state_add(pcp->stat_threshold, zone, item);
> }
>
> void inc_zone_state(struct zone *zone, enum zone_stat_item item)
> {
> 	unsigned long flags;
>
> 	percpu_local_irqsave(flags);
> 	__inc_zone_state(zone, item);
> 	percpu_local_irqrestore(flags);
> }
>
> Which is more compact and does not duplicate code.

This is almost like the current code. But lets avoid percpu_local_irqs
etc if we can.
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