[patch 2/2]compaction: check lock contention first before taking lock

[Shaohua Li <shli@kernel.org>]

isolate_migratepages_range will take zone->lru_lock first and check if the lock
is contented, if yes, it will release the lock. This isn't efficient. If the
lock is truly contented, a lock/unlock pair will increase the lock contention.
We'd better check if the lock is contended first. compact_trylock_irqsave
perfectly meets the requirement.

Signed-off-by: Shaohua Li <shli@fusionio.com>
---
 mm/compaction.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

Index: linux/mm/compaction.c
===================================================================
--- linux.orig/mm/compaction.c	2012-09-06 14:46:13.923144263 +0800
+++ linux/mm/compaction.c	2012-09-06 14:46:58.118588574 +0800
@@ -295,9 +295,9 @@ isolate_migratepages_range(struct zone *
 	}
 
 	/* Time to isolate some pages for migration */
-	cond_resched();
-	spin_lock_irqsave(&zone->lru_lock, flags);
-	locked = true;
+	locked = compact_trylock_irqsave(&zone->lru_lock, &flags, cc);
+	if (!locked)
+		goto skip;
 	for (; low_pfn < end_pfn; low_pfn++) {
 		struct page *page;
 
@@ -400,6 +400,7 @@ isolate_migratepages_range(struct zone *
 	if (locked)
 		spin_unlock_irqrestore(&zone->lru_lock, flags);
 
+skip:
 	trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated);
 
 	if (!nr_isolated)

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