On Fri, Jun 20, 2014 at 05:49:37PM +0200, Vlastimil Babka wrote:
> Compaction scanners try to lock zone locks as late as possible by checking
> many page or pageblock properties opportunistically without lock and skipping
> them if not unsuitable. For pages that pass the initial checks, some properties
> have to be checked again safely under lock. However, if the lock was already
> held from a previous iteration in the initial checks, the rechecks are
> unnecessary.
>
> This patch therefore skips the rechecks when the lock was already held. This is
> now possible to do, since we don't (potentially) drop and reacquire the lock
> between the initial checks and the safe rechecks anymore.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
Reviewed-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> ---
> mm/compaction.c | 53 +++++++++++++++++++++++++++++++----------------------
> 1 file changed, 31 insertions(+), 22 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 40da812..9f6e857 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -324,22 +324,30 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> goto isolate_fail;
>
> /*
> - * The zone lock must be held to isolate freepages.
> - * Unfortunately this is a very coarse lock and can be
> - * heavily contended if there are parallel allocations
> - * or parallel compactions. For async compaction do not
> - * spin on the lock and we acquire the lock as late as
> - * possible.
> + * If we already hold the lock, we can skip some rechecking.
> + * Note that if we hold the lock now, checked_pageblock was
> + * already set in some previous iteration (or strict is true),
> + * so it is correct to skip the suitable migration target
> + * recheck as well.
> */
> - if (!locked)
> + if (!locked) {
> + /*
> + * The zone lock must be held to isolate freepages.
> + * Unfortunately this is a very coarse lock and can be
> + * heavily contended if there are parallel allocations
> + * or parallel compactions. For async compaction do not
> + * spin on the lock and we acquire the lock as late as
> + * possible.
> + */
> locked = compact_trylock_irqsave(&cc->zone->lock,
> &flags, cc);
> - if (!locked)
> - break;
> + if (!locked)
> + break;
>
> - /* Recheck this is a buddy page under lock */
> - if (!PageBuddy(page))
> - goto isolate_fail;
> + /* Recheck this is a buddy page under lock */
> + if (!PageBuddy(page))
> + goto isolate_fail;
> + }
>
> /* Found a free page, break it into order-0 pages */
> isolated = split_free_page(page);
> @@ -623,19 +631,20 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> page_count(page) > page_mapcount(page))
> continue;
>
> - /* If the lock is not held, try to take it */
> - if (!locked)
> + /* If we already hold the lock, we can skip some rechecking */
> + if (!locked) {
> locked = compact_trylock_irqsave(&zone->lru_lock,
> &flags, cc);
> - if (!locked)
> - break;
> + if (!locked)
> + break;
>
> - /* Recheck PageLRU and PageTransHuge under lock */
> - if (!PageLRU(page))
> - continue;
> - if (PageTransHuge(page)) {
> - low_pfn += (1 << compound_order(page)) - 1;
> - continue;
> + /* Recheck PageLRU and PageTransHuge under lock */
> + if (!PageLRU(page))
> + continue;
> + if (PageTransHuge(page)) {
> + low_pfn += (1 << compound_order(page)) - 1;
> + continue;
> + }
> }
>
> lruvec = mem_cgroup_page_lruvec(page, zone);
> --
> 1.8.4.5
>
> --
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