On 06/20/2014 11:49 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 <vbabka@xxxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx>
Reviewed-by: Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx>
> ---
> mm/compaction.c | 40 +++++++++++++++++++++++++++++++---------
> 1 file changed, 31 insertions(+), 9 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 9f6e857..41c7005 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -287,7 +287,7 @@ 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)
> @@ -296,6 +296,7 @@ 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);
>
> @@ -369,6 +370,9 @@ isolate_fail:
> break;
> }
>
> + /* Record how far we have got within the block */
> + *start_pfn = blockpfn;
> +
> trace_mm_compaction_isolate_freepages(nr_scanned, total_isolated);
>
> /*
> @@ -413,6 +417,9 @@ 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;
>
> @@ -423,8 +430,8 @@ 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
> @@ -708,6 +715,7 @@ static void isolate_freepages(struct zone *zone,
> {
> struct page *page;
> unsigned long block_start_pfn; /* start of current pageblock */
> + unsigned long isolate_start_pfn; /* exact pfn we start at */
> unsigned long block_end_pfn; /* end of current pageblock */
> unsigned long low_pfn; /* lowest pfn scanner is able to scan */
> int nr_freepages = cc->nr_freepages;
> @@ -716,14 +724,15 @@ static void isolate_freepages(struct zone *zone,
> /*
> * Initialise the free scanner. The starting point is where we last
> * successfully isolated from, zone-cached value, or the end of the
> - * zone when isolating for the first time. We need this aligned to
> - * the pageblock boundary, because we do
> + * zone when isolating for the first time. For looping we also need
> + * this pfn aligned down to the pageblock boundary, because we do
> * block_start_pfn -= pageblock_nr_pages in the for loop.
> * For ending point, take care when isolating in last pageblock of a
> * a zone which ends in the middle of a pageblock.
> * The low boundary is the end of the pageblock the migration scanner
> * is using.
> */
> + isolate_start_pfn = cc->free_pfn;
> block_start_pfn = cc->free_pfn & ~(pageblock_nr_pages-1);
> block_end_pfn = min(block_start_pfn + pageblock_nr_pages,
> zone_end_pfn(zone));
> @@ -736,7 +745,8 @@ static void isolate_freepages(struct zone *zone,
> */
> for (; block_start_pfn >= low_pfn && cc->nr_migratepages > nr_freepages;
> block_end_pfn = block_start_pfn,
> - block_start_pfn -= pageblock_nr_pages) {
> + block_start_pfn -= pageblock_nr_pages,
> + isolate_start_pfn = block_start_pfn) {
> unsigned long isolated;
>
> /*
> @@ -770,13 +780,25 @@ static void isolate_freepages(struct zone *zone,
> if (!isolation_suitable(cc, page))
> continue;
>
> - /* Found a block suitable for isolating free pages from */
> - cc->free_pfn = block_start_pfn;
> - isolated = isolate_freepages_block(cc, block_start_pfn,
> + /* Found a block suitable for isolating free pages from. */
> + 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,
> + * which is where isolate_freepages_block() left off.
> + * But if it scanned the whole pageblock, isolate_start_pfn
> + * now points at block_end_pfn, which is the start of the next
> + * pageblock.
> + * In that case we will however want to restart at the start
> + * of the previous pageblock.
> + */
> + cc->free_pfn = (isolate_start_pfn < block_end_pfn) ?
> + isolate_start_pfn :
> + block_start_pfn - pageblock_nr_pages;
> +
> + /*
> * 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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