On 06/09/2014 05:26 PM, Vlastimil Babka wrote:
> Unlike the migration scanner, the free scanner remembers the beginning of the
> last scanned pageblock in cc->free_pfn. It might be therefore rescanning pages
> uselessly when called several times during single compaction. This might have
> been useful when pages were returned to the buddy allocator after a failed
> migration, but this is no longer the case.
>
> This patch changes the meaning of cc->free_pfn so that if it points to a
> middle of a pageblock, that pageblock is scanned only from cc->free_pfn to the
> end. isolate_freepages_block() will record the pfn of the last page it looked
> at, which is then used to update cc->free_pfn.
>
> In the mmtests stress-highalloc benchmark, this has resulted in lowering the
> ratio between pages scanned by both scanners, from 2.5 free pages per migrate
> page, to 2.25 free pages per migrate page, without affecting success rates.
>
> Signed-off-by: Vlastimil Babka <vba...@suse.cz>
Reviewed-by: Zhang Yanfei <zhangyan...@cn.fujitsu.com>
> Cc: Minchan Kim <minc...@kernel.org>
> Cc: Mel Gorman <mgor...@suse.de>
> Cc: Joonsoo Kim <iamjoonsoo....@lge.com>
> Cc: Michal Nazarewicz <min...@mina86.com>
> Cc: Naoya Horiguchi <n-horigu...@ah.jp.nec.com>
> Cc: Christoph Lameter <c...@linux.com>
> Cc: Rik van Riel <r...@redhat.com>
> Cc: David Rientjes <rient...@google.com>
> ---
> mm/compaction.c | 33 ++++++++++++++++++++++++++++-----
> 1 file changed, 28 insertions(+), 5 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 83f72bd..58dfaaa 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -297,7 +297,7 @@ static bool suitable_migration_target(struct page *page)
> * (even though it may still end up isolating some pages).
> */
> static unsigned long isolate_freepages_block(struct compact_control *cc,
> - unsigned long blockpfn,
> + unsigned long *start_pfn,
> unsigned long end_pfn,
> struct list_head *freelist,
> bool strict)
> @@ -306,6 +306,7 @@ static unsigned long isolate_freepages_block(struct
> compact_control *cc,
> struct page *cursor, *valid_page = NULL;
> unsigned long flags;
> bool locked = false;
> + unsigned long blockpfn = *start_pfn;
>
> cursor = pfn_to_page(blockpfn);
>
> @@ -314,6 +315,9 @@ static unsigned long isolate_freepages_block(struct
> compact_control *cc,
> int isolated, i;
> struct page *page = cursor;
>
> + /* Record how far we have got within the block */
> + *start_pfn = blockpfn;
> +
> /*
> * Periodically drop the lock (if held) regardless of its
> * contention, to give chance to IRQs. Abort async compaction
> @@ -424,6 +428,9 @@ isolate_freepages_range(struct compact_control *cc,
> LIST_HEAD(freelist);
>
> for (pfn = start_pfn; pfn < end_pfn; pfn += isolated) {
> + /* Protect pfn from changing by isolate_freepages_block */
> + unsigned long isolate_start_pfn = pfn;
> +
> if (!pfn_valid(pfn) || cc->zone != page_zone(pfn_to_page(pfn)))
> break;
>
> @@ -434,8 +441,8 @@ isolate_freepages_range(struct compact_control *cc,
> block_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
> block_end_pfn = min(block_end_pfn, end_pfn);
>
> - isolated = isolate_freepages_block(cc, pfn, block_end_pfn,
> - &freelist, true);
> + isolated = isolate_freepages_block(cc, &isolate_start_pfn,
> + block_end_pfn, &freelist, true);
>
> /*
> * In strict mode, isolate_freepages_block() returns 0 if
> @@ -774,6 +781,7 @@ static void isolate_freepages(struct zone *zone,
> block_end_pfn = block_start_pfn,
> block_start_pfn -= pageblock_nr_pages) {
> unsigned long isolated;
> + unsigned long isolate_start_pfn;
>
> /*
> * This can iterate a massively long zone without finding any
> @@ -807,12 +815,27 @@ static void isolate_freepages(struct zone *zone,
> continue;
>
> /* Found a block suitable for isolating free pages from */
> - cc->free_pfn = block_start_pfn;
> - isolated = isolate_freepages_block(cc, block_start_pfn,
> + isolate_start_pfn = block_start_pfn;
> +
> + /*
> + * If we are restarting the free scanner in this block, do not
> + * rescan the beginning of the block
> + */
> + if (cc->free_pfn < block_end_pfn)
> + isolate_start_pfn = cc->free_pfn;
> +
> + isolated = isolate_freepages_block(cc, &isolate_start_pfn,
> block_end_pfn, freelist, false);
> nr_freepages += isolated;
>
> /*
> + * Remember where the free scanner should restart next time.
> + * This will point to the last page of pageblock we just
> + * scanned, if we scanned it fully.
> + */
> + cc->free_pfn = isolate_start_pfn;
> +
> + /*
> * 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
>
--
Thanks.
Zhang Yanfei
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