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

[Vlastimil Babka <vbabka@suse.cz>]

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@suse.cz>
>> Cc: Minchan Kim <minchan@kernel.org>
>> Cc: Mel Gorman <mgorman@suse.de>
>> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>> Cc: Christoph Lameter <cl@linux.com>
>> Cc: Rik van Riel <riel@redhat.com>
>> ---
>>   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
>