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?
Thanks,
Naoya Horiguchi
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