[patch 3/3]vmscan: cleanup kswapd_try_to_sleep

[Shaohua Li <shaohua.li@intel.com>]

cleanup kswapd_try_to_sleep() a little bit. Sometimes kswapd doesn't
really sleep. In such case, don't call prepare_to_wait/finish_wait.
It just wastes CPU.

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

Index: linux/mm/vmscan.c
===================================================================
--- linux.orig/mm/vmscan.c	2011-07-28 15:52:35.000000000 +0800
+++ linux/mm/vmscan.c	2011-07-28 15:55:56.000000000 +0800
@@ -2709,13 +2709,11 @@ static void kswapd_try_to_sleep(pg_data_
 	if (freezing(current) || kthread_should_stop())
 		return;
 
-	prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
-
 	/* Try to sleep for a short interval */
 	if (!sleeping_prematurely(pgdat, order, remaining, classzone_idx)) {
+		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
 		remaining = schedule_timeout(HZ/10);
 		finish_wait(&pgdat->kswapd_wait, &wait);
-		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
 	}
 
 	/*
@@ -2734,7 +2732,9 @@ static void kswapd_try_to_sleep(pg_data_
 		 * them before going back to sleep.
 		 */
 		set_pgdat_percpu_threshold(pgdat, calculate_normal_threshold);
+		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
 		schedule();
+		finish_wait(&pgdat->kswapd_wait, &wait);
 		set_pgdat_percpu_threshold(pgdat, calculate_pressure_threshold);
 	} else {
 		if (remaining)
@@ -2742,7 +2742,6 @@ static void kswapd_try_to_sleep(pg_data_
 		else
 			count_vm_event(KSWAPD_HIGH_WMARK_HIT_QUICKLY);
 	}
-	finish_wait(&pgdat->kswapd_wait, &wait);
 }
 
 /*


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