linux-kernel - Re: [PATCH 6/6] mm: page_alloc: Reduce cost of the fair zone allocation policy

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Date:	Mon, 11 Aug 2014 13:12:42 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 6/6] mm: page_alloc: Reduce cost of the fair zone
 allocation policy

On Fri, Aug 08, 2014 at 05:27:15PM +0200, Vlastimil Babka wrote:
> On 07/09/2014 10:13 AM, Mel Gorman wrote:
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1604,6 +1604,9 @@ again:
> >  	}
> >  
> >  	__mod_zone_page_state(zone, NR_ALLOC_BATCH, -(1 << order));
> 
> This can underflow zero, right?
> 

Yes, because of per-cpu accounting drift.

> > +	if (zone_page_state(zone, NR_ALLOC_BATCH) == 0 &&
> 
> AFAICS, zone_page_state will correct negative values to zero only for
> CONFIG_SMP. Won't this check be broken on !CONFIG_SMP?
> 

On !CONFIG_SMP how can there be per-cpu accounting drift that would make
that counter negative?

-- 
Mel Gorman
SUSE Labs
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