Hi Mel, I have a question below. On Fri, Sep 21, 2012 at 11:46:19AM +0100, Mel Gorman wrote: > Compactions migrate scanner acquires the zone->lru_lock when scanning a range > of pages looking for LRU pages to acquire. It does this even if there are > no LRU pages in the range. If multiple processes are compacting then this > can cause severe locking contention. To make matters worse commit b2eef8c0 > (mm: compaction: minimise the time IRQs are disabled while isolating pages > for migration) releases the lru_lock every SWAP_CLUSTER_MAX pages that are > scanned. > > This patch makes two changes to how the migrate scanner acquires the LRU > lock. First, it only releases the LRU lock every SWAP_CLUSTER_MAX pages if > the lock is contended. This reduces the number of times it unnecessarily > disables and re-enables IRQs. The second is that it defers acquiring the > LRU lock for as long as possible. If there are no LRU pages or the only > LRU pages are transhuge then the LRU lock will not be acquired at all > which reduces contention on zone->lru_lock. > > Signed-off-by: Mel Gorman <mgorman@xxxxxxx> > Acked-by: Rik van Riel <riel@xxxxxxxxxx> > --- > mm/compaction.c | 63 +++++++++++++++++++++++++++++++++++++------------------ > 1 file changed, 43 insertions(+), 20 deletions(-) > > diff --git a/mm/compaction.c b/mm/compaction.c > index 6b55491..a6068ff 100644 > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -50,6 +50,11 @@ static inline bool migrate_async_suitable(int migratetype) > return is_migrate_cma(migratetype) || migratetype == MIGRATE_MOVABLE; > } > > +static inline bool should_release_lock(spinlock_t *lock) > +{ > + return need_resched() || spin_is_contended(lock); > +} > + > /* > * Compaction requires the taking of some coarse locks that are potentially > * very heavily contended. Check if the process needs to be scheduled or > @@ -62,7 +67,7 @@ static inline bool migrate_async_suitable(int migratetype) > static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags, > bool locked, struct compact_control *cc) > { > - if (need_resched() || spin_is_contended(lock)) { > + if (should_release_lock(lock)) { > if (locked) { > spin_unlock_irqrestore(lock, *flags); > locked = false; > @@ -327,7 +332,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, > isolate_mode_t mode = 0; > struct lruvec *lruvec; > unsigned long flags; > - bool locked; > + bool locked = false; > > /* > * Ensure that there are not too many pages isolated from the LRU > @@ -347,23 +352,17 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, > > /* Time to isolate some pages for migration */ > cond_resched(); > - spin_lock_irqsave(&zone->lru_lock, flags); > - locked = true; > for (; low_pfn < end_pfn; low_pfn++) { > struct page *page; > > /* give a chance to irqs before checking need_resched() */ > - if (!((low_pfn+1) % SWAP_CLUSTER_MAX)) { > - spin_unlock_irqrestore(&zone->lru_lock, flags); > - locked = false; > + if (locked && !((low_pfn+1) % SWAP_CLUSTER_MAX)) { > + if (should_release_lock(&zone->lru_lock)) { > + spin_unlock_irqrestore(&zone->lru_lock, flags); > + locked = false; > + } > } > > - /* Check if it is ok to still hold the lock */ > - locked = compact_checklock_irqsave(&zone->lru_lock, &flags, > - locked, cc); > - if (!locked || fatal_signal_pending(current)) > - break; > - > /* > * migrate_pfn does not necessarily start aligned to a > * pageblock. Ensure that pfn_valid is called when moving > @@ -403,21 +402,38 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, > pageblock_nr = low_pfn >> pageblock_order; > if (!cc->sync && last_pageblock_nr != pageblock_nr && > !migrate_async_suitable(get_pageblock_migratetype(page))) { > - low_pfn += pageblock_nr_pages; > - low_pfn = ALIGN(low_pfn, pageblock_nr_pages) - 1; > - last_pageblock_nr = pageblock_nr; > - continue; > + goto next_pageblock; > } > > + /* Check may be lockless but that's ok as we recheck later */ > if (!PageLRU(page)) > continue; > > /* > - * PageLRU is set, and lru_lock excludes isolation, > - * splitting and collapsing (collapsing has already > - * happened if PageLRU is set). > + * PageLRU is set. lru_lock normally excludes isolation > + * splitting and collapsing (collapsing has already happened > + * if PageLRU is set) but the lock is not necessarily taken > + * here and it is wasteful to take it just to check transhuge. > + * Check transhuge without lock and skip if it's either a > + * transhuge or hugetlbfs page. > */ > if (PageTransHuge(page)) { > + if (!locked) > + goto next_pageblock; Why skip all pages in a pageblock if !locked? Shouldn't we add some comment? > + low_pfn += (1 << compound_order(page)) - 1; > + continue; > + } > + > + /* Check if it is ok to still hold the lock */ > + locked = compact_checklock_irqsave(&zone->lru_lock, &flags, > + locked, cc); > + if (!locked || fatal_signal_pending(current)) > + break; > + > + /* Recheck PageLRU and PageTransHuge under lock */ > + if (!PageLRU(page)) > + continue; > + if (PageTransHuge(page)) { > low_pfn += (1 << compound_order(page)) - 1; > continue; > } > @@ -444,6 +460,13 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, > ++low_pfn; > break; > } > + > + continue; > + > +next_pageblock: > + low_pfn += pageblock_nr_pages; > + low_pfn = ALIGN(low_pfn, pageblock_nr_pages) - 1; > + last_pageblock_nr = pageblock_nr; > } > > acct_isolated(zone, locked, cc); > -- > 1.7.9.2 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a> -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html