mm: compaction: check lock contention first before taking lock

mm: compaction: check lock contention first before taking lock

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>
Acked-by: Mel Gorman <mgorman@suse.de>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>