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

[Joonsoo Kim]

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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