Re: [PATCH 2/2] mm/compaction: avoid rescanning pageblocks in isolate_freepages

[Vlastimil Babka]

On 05/07/2014 12:19 AM, Naoya Horiguchi wrote:
> On Fri, May 02, 2014 at 05:27:55PM +0200, Vlastimil Babka wrote:
> > The compaction free scanner in isolate_freepages() currently remembers PFN of
> > the highest pageblock where it successfully isolates, to be used as the
> > starting pageblock for the next invocation. The rationale behind this is that
> > page migration might return free pages to the allocator when migration fails
> > and we don't want to skip them if the compaction continues.
> >
> > Since migration now returns free pages back to compaction code where they can
> > be reused, this is no longer a concern. This patch changes isolate_freepages()
> > so that the PFN for restarting is updated with each pageblock where isolation
> > is attempted. Using stress-highalloc from mmtests, this resulted in 10%
> > reduction of the pages scanned by the free scanner.
> >
> > Note that the somewhat similar functionality that records highest successful
> > pageblock in zone->compact_cached_free_pfn, remains unchanged. This cache is
> > used when the whole compaction is restarted, not for multiple invocations of
> > the free scanner during single compaction.
> >
> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxx>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > Cc: Christoph Lameter <cl@xxxxxxxxx>
> > Cc: Rik van Riel <riel@xxxxxxxxxx>
> > ---
> >   mm/compaction.c | 18 ++++++------------
> >   1 file changed, 6 insertions(+), 12 deletions(-)
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index 873d7de..1967850 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -686,12 +686,6 @@ static void isolate_freepages(struct zone *zone,
> >   	low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages);
> >
> >   	/*
> > -	 * If no pages are isolated, the block_start_pfn < low_pfn check
> > -	 * will kick in.
> > -	 */
> > -	next_free_pfn = 0;
> > -
> > -	/*
> >   	 * Isolate free pages until enough are available to migrate the
> >   	 * pages on cc->migratepages. We stop searching if the migrate
> >   	 * and free page scanners meet or enough free pages are isolated.
> > @@ -731,19 +725,19 @@ static void isolate_freepages(struct zone *zone,
> >   			continue;
> >
> >   		/* Found a block suitable for isolating free pages from */
> > +		next_free_pfn = block_start_pfn;
> >   		isolated = isolate_freepages_block(cc, block_start_pfn,
> >   					block_end_pfn, freelist, false);
> >   		nr_freepages += isolated;
> >
> >   		/*
> > -		 * Record the highest PFN we isolated pages from. When next
> > -		 * looking for free pages, the search will restart here as
> > -		 * page migration may have returned some pages to the allocator
> > +		 * Set a flag that we successfully isolated in this pageblock.
> > +		 * In the next loop iteration, zone->compact_cached_free_pfn
> > +		 * will not be updated and thus it will effectively contain the
> > +		 * highest pageblock we isolated pages from.
> >   		 */
> > -		if (isolated && next_free_pfn == 0) {
> > +		if (isolated)
> >   			cc->finished_update_free = true;
> > -			next_free_pfn = block_start_pfn;
> > -		}
>
> Why don't you completely remove next_free_pfn and update cc->free_pfn directly?

Hi,

well you could ask the same about the nr_freepages variable. For me 
personally local variable (now with a comment for next_free_pfn) looks 
more readable. The function currently received cleanup from several 
people (including me), so I wouldn't want to change it again, unless 
others also think it would be better. From compiler standpoint both 
variants should be the same I guess.

Vlastimil

> Thanks,
> Naoya Horiguchi

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