linux-kernel - Re: [PATCH v5 1/4] mm/page_alloc: fix incorrect isolation behavior by rechecking migratetype

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Date:	Tue, 18 Nov 2014 12:11:26 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Weijie Yang <weijie.yang.kh@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan@...nel.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Tang Chen <tangchen@...fujitsu.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Wen Congyang <wency@...fujitsu.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Heesub Shin <heesub.shin@...sung.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Ritesh Harjani <ritesh.list@...il.com>,
	t.stanislaws@...sung.com, Gioh Kim <gioh.kim@....com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v5 1/4] mm/page_alloc: fix incorrect isolation behavior
 by rechecking migratetype

On Fri, Nov 14, 2014 at 10:33:01AM +0000, Mel Gorman wrote:
> > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > > index 4593567..3d090af 100644
> > > --- a/include/linux/mmzone.h
> > > +++ b/include/linux/mmzone.h
> > > @@ -431,6 +431,15 @@ struct zone {
> > >          */
> > >         int                     nr_migrate_reserve_block;
> > >
> > > +#ifdef CONFIG_MEMORY_ISOLATION
> > > +       /*
> > > +        * Number of isolated pageblock. It is used to solve incorrect
> > > +        * freepage counting problem due to racy retrieving migratetype
> > > +        * of pageblock. Protected by zone->lock.
> > > +        */
> > > +       unsigned long           nr_isolate_pageblock;
> > > +#endif
> > > +
> > 
> > First sorry for this deferred reply, I see these patches have been merged
> > into the mainline.
> > However, I still have a tiny question:
> > Why use ZONE_PADDING(_pad1_)  seperate it and zone->lock?
> > How about move it to the same cacheline with zone->lock, because it is
> > accessed under zone->lock?
> > 
> 
> zone->lock is currently sharing lines with the data that is frequently
> updated under zone lock and some of the dirty data cache line bouncing has
> completed when the lock is acquired. nr_isolate_pageblock is a read-mostly
> field and in some cases will never be used. It's fine where it is beside
> other read-mostly fields.
> 

My bad...
I don't remember why I decide that place. :/

It seems better to move nr_isolate_pageblock to the same cacheline with
zone->lock, but, as Mel said, it is rarely used field so improvement would
be marginal.

Thanks.
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