Re: [PATCH v5 02/14] mm, compaction: defer each zone individually instead of preferred zone

[Vlastimil Babka]

On 07/29/2014 11:12 AM, Vlastimil Babka wrote:
> On 07/29/2014 08:38 AM, Joonsoo Kim wrote:
> > >    /* Return values for compact_zone() and try_to_compact_pages() */
> > > +/* compaction didn't start as it was deferred due to past failures */
> > > +#define COMPACT_DEFERRED	0
> > >    /* compaction didn't start as it was not possible or direct reclaim was more suitable */
> > > -#define COMPACT_SKIPPED		0
> > > +#define COMPACT_SKIPPED		1
> >
> > Hello,
> >
> > This change makes some users of compaction_suitable() failed
> > unintentionally, because they assume that COMPACT_SKIPPED is 0.
> > Please fix them according to this change.
>
> Oops, good catch. Thanks!
>
> > > @@ -2324,27 +2327,31 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> > >    				order, zonelist, high_zoneidx,
> > >    				alloc_flags & ~ALLOC_NO_WATERMARKS,
> > >    				preferred_zone, classzone_idx, migratetype);
> > > +
> > >    		if (page) {
> > > -			preferred_zone->compact_blockskip_flush = false;
> > > -			compaction_defer_reset(preferred_zone, order, true);
> > > +			struct zone *zone = page_zone(page);
> > > +
> > > +			zone->compact_blockskip_flush = false;
> > > +			compaction_defer_reset(zone, order, true);
> > >    			count_vm_event(COMPACTSUCCESS);
> > >    			return page;
> > >    		}
> > >
> > >    		/*
> > > +		 * last_compact_zone is where try_to_compact_pages thought
> > > +		 * allocation should succeed, so it did not defer compaction.
> > > +		 * But now we know that it didn't succeed, so we do the defer.
> > > +		 */
> > > +		if (last_compact_zone && mode != MIGRATE_ASYNC)
> > > +			defer_compaction(last_compact_zone, order);
> >
> > I still don't understand why defer_compaction() is needed here.
> > defer_compaction() is intended for not struggling doing compaction on
> > the zone where we already have tried compaction and found that it
> > isn't suitable for compaction. Allocation failure doesn't tell us
> > that we have tried compaction for all the zone range so we shouldn't
> > make a decision here to defer compaction on this zone carelessly.
>
> OK I can remove that, it should make the code nicer anyway.

Weird, that removal of this defer_compaction() call seems ho have 
quadrupled compact_stall and compact_fail counts. The scanner pages 
counters however increased by only 10% so that could indicate the 
problem is occuring only in a small zone such as DMA. Could be another 
case of mismatch between watermark checking in compaction and 
allocation? Perhaps the lack of proper classzone_idx in the compaction 
check? Sigh.

> I also agree
> with the argument "for all the zone range" and I also realized that it's
> not (both before and after this patch) really the case. I planned to fix
> that in the future, but I can probably do it now.
> The plan is to call defer_compaction() only when compaction returned
> COMPACT_COMPLETE (and not COMPACT_PARTIAL) as it means the whole zone
> was scanned. Otherwise there will be bias towards the beginning of the
> zone in the migration scanner - compaction will be deferred half-way and
> then cached pfn's might be reset when it restarts, and the rest of the
> zone won't be scanned at all.

Hm despite expectations, this didn't seem to make much difference. But 
maybe there will be once I have some idea what happened to those stalls.

> > Thanks.

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