Re: [PATCH v4 3/3] mm/compaction: enhance compaction finish condition

[Joonsoo Kim]

On Wed, Feb 18, 2015 at 04:04:05PM -0800, Andrew Morton wrote:
> On Thu, 12 Feb 2015 16:15:05 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> 
> > Compaction has anti fragmentation algorithm. It is that freepage
> > should be more than pageblock order to finish the compaction if we don't
> > find any freepage in requested migratetype buddy list. This is for
> > mitigating fragmentation, but, there is a lack of migratetype
> > consideration and it is too excessive compared to page allocator's anti
> > fragmentation algorithm.
> > 
> > Not considering migratetype would cause premature finish of compaction.
> > For example, if allocation request is for unmovable migratetype,
> > freepage with CMA migratetype doesn't help that allocation and
> > compaction should not be stopped. But, current logic regards this
> > situation as compaction is no longer needed, so finish the compaction.
> > 
> > Secondly, condition is too excessive compared to page allocator's logic.
> > We can steal freepage from other migratetype and change pageblock
> > migratetype on more relaxed conditions in page allocator. This is designed
> > to prevent fragmentation and we can use it here. Imposing hard constraint
> > only to the compaction doesn't help much in this case since page allocator
> > would cause fragmentation again.
> > 
> > To solve these problems, this patch borrows anti fragmentation logic from
> > page allocator. It will reduce premature compaction finish in some cases
> > and reduce excessive compaction work.
> > 
> > stress-highalloc test in mmtests with non movable order 7 allocation shows
> > considerable increase of compaction success rate.
> > 
> > Compaction success rate (Compaction success * 100 / Compaction stalls, %)
> > 31.82 : 42.20
> > 
> > I tested it on non-reboot 5 runs stress-highalloc benchmark and found that
> > there is no more degradation on allocation success rate than before. That
> > roughly means that this patch doesn't result in more fragmentations.
> > 
> > Vlastimil suggests additional idea that we only test for fallbacks
> > when migration scanner has scanned a whole pageblock. It looked good for
> > fragmentation because chance of stealing increase due to making more
> > free pages in certain pageblock. So, I tested it, but, it results in
> > decreased compaction success rate, roughly 38.00. I guess the reason that
> > if system is low memory condition, watermark check could be failed due to
> > not enough order 0 free page and so, sometimes, we can't reach a fallback
> > check although migrate_pfn is aligned to pageblock_nr_pages. I can insert
> > code to cope with this situation but it makes code more complicated so
> > I don't include his idea at this patch.
> > 
> > ...
> >
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -1170,13 +1170,23 @@ static int __compact_finished(struct zone *zone, struct compact_control *cc,
> >  	/* Direct compactor: Is a suitable page free? */
> >  	for (order = cc->order; order < MAX_ORDER; order++) {
> >  		struct free_area *area = &zone->free_area[order];
> > +		bool can_steal;
> >  
> >  		/* Job done if page is free of the right migratetype */
> >  		if (!list_empty(&area->free_list[migratetype]))
> >  			return COMPACT_PARTIAL;
> >  
> > -		/* Job done if allocation would set block type */
> > -		if (order >= pageblock_order && area->nr_free)
> > +		/* MIGRATE_MOVABLE can fallback on MIGRATE_CMA */
> > +		if (migratetype == MIGRATE_MOVABLE &&
> > +			!list_empty(&area->free_list[MIGRATE_CMA]))
> > +			return COMPACT_PARTIAL;
> 
> MIGRATE_CMA isn't defined if CONFIG_CMA=n.
> 
> --- a/mm/compaction.c~mm-compaction-enhance-compaction-finish-condition-fix
> +++ a/mm/compaction.c
> @@ -1180,11 +1180,12 @@ static int __compact_finished(struct zon
>  		if (!list_empty(&area->free_list[migratetype]))
>  			return COMPACT_PARTIAL;
>  
> +#ifdef CONFIG_CMA
>  		/* MIGRATE_MOVABLE can fallback on MIGRATE_CMA */
>  		if (migratetype == MIGRATE_MOVABLE &&
>  			!list_empty(&area->free_list[MIGRATE_CMA]))
>  			return COMPACT_PARTIAL;
> -
> +#endif
>  		/*
>  		 * Job done if allocation would steal freepages from
>  		 * other migratetype buddy lists.
> 
> Please review the rest of the patchset for the CONFIG_CMA=n case (is it
> all necessary?), runtime test it and let me know?

Okay.

Following is update version which solves compile issue for the
CONFIG_CMA=n and has minor clean-up pointed by Vlastimil.

I did runtime tests for both CONFIG_CMA=y and CONFIG_CMA=n cases and it
was fine. :)

Thanks.

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