vmscan: factor out kswapd sleeping logic from kswapd()

Currently, kswapd() has deep nesting and is slightly hard to read.  Clean
this up.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2371,6 +2371,50 @@ out:
 	return sc.nr_reclaimed;
 }
 
+static void kswapd_try_to_sleep(pg_data_t *pgdat, int order)
+{
+	long remaining = 0;
+	DEFINE_WAIT(wait);
+
+	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)) {
+		remaining = schedule_timeout(HZ/10);
+		finish_wait(&pgdat->kswapd_wait, &wait);
+		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
+	}
+
+	/*
+	 * After a short sleep, check if it was a premature sleep. If not, then
+	 * go fully to sleep until explicitly woken up.
+	 */
+	if (!sleeping_prematurely(pgdat, order, remaining)) {
+		trace_mm_vmscan_kswapd_sleep(pgdat->node_id);
+
+		/*
+		 * vmstat counters are not perfectly accurate and the estimated
+		 * value for counters such as NR_FREE_PAGES can deviate from the
+		 * true value by nr_online_cpus * threshold. To avoid the zone
+		 * watermarks being breached while under pressure, we reduce the
+		 * per-cpu vmstat threshold while kswapd is awake and restore
+		 * them before going back to sleep.
+		 */
+		set_pgdat_percpu_threshold(pgdat, calculate_normal_threshold);
+		schedule();
+		set_pgdat_percpu_threshold(pgdat, calculate_pressure_threshold);
+	} else {
+		if (remaining)
+			count_vm_event(KSWAPD_LOW_WMARK_HIT_QUICKLY);
+		else
+			count_vm_event(KSWAPD_HIGH_WMARK_HIT_QUICKLY);
+	}
+	finish_wait(&pgdat->kswapd_wait, &wait);
+}
+
 /*
  * The background pageout daemon, started as a kernel thread
  * from the init process.
@@ -2389,7 +2433,7 @@ static int kswapd(void *p)
 	unsigned long order;
 	pg_data_t *pgdat = (pg_data_t*)p;
 	struct task_struct *tsk = current;
-	DEFINE_WAIT(wait);
+
 	struct reclaim_state reclaim_state = {
 		.reclaimed_slab = 0,
 	};
@@ -2421,7 +2465,6 @@ static int kswapd(void *p)
 		unsigned long new_order;
 		int ret;
 
-		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
 		new_order = pgdat->kswapd_max_order;
 		pgdat->kswapd_max_order = 0;
 		if (order < new_order) {
@@ -2431,52 +2474,9 @@ static int kswapd(void *p)
 			 */
 			order = new_order;
 		} else {
-			if (!freezing(current) && !kthread_should_stop()) {
-				long remaining = 0;
-
-				/* Try to sleep for a short interval */
-				if (!sleeping_prematurely(pgdat, order, remaining)) {
-					remaining = schedule_timeout(HZ/10);
-					finish_wait(&pgdat->kswapd_wait, &wait);
-					prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
-				}
-
-				/*
-				 * After a short sleep, check if it was a
-				 * premature sleep. If not, then go fully
-				 * to sleep until explicitly woken up
-				 */
-				if (!sleeping_prematurely(pgdat, order, remaining)) {
-					trace_mm_vmscan_kswapd_sleep(pgdat->node_id);
-
-					/*
-					 * vmstat counters are not perfectly
-					 * accurate and the estimated value
-					 * for counters such as NR_FREE_PAGES
-					 * can deviate from the true value by
-					 * nr_online_cpus * threshold. To
-					 * avoid the zone watermarks being
-					 * breached while under pressure, we
-					 * reduce the per-cpu vmstat threshold
-					 * while kswapd is awake and restore
-					 * them before going back to sleep.
-					 */
-					set_pgdat_percpu_threshold(pgdat,
-						calculate_normal_threshold);
-					schedule();
-					set_pgdat_percpu_threshold(pgdat,
-						calculate_pressure_threshold);
-				} else {
-					if (remaining)
-						count_vm_event(KSWAPD_LOW_WMARK_HIT_QUICKLY);
-					else
-						count_vm_event(KSWAPD_HIGH_WMARK_HIT_QUICKLY);
-				}
-			}
-
+			kswapd_try_to_sleep(pgdat, order);
 			order = pgdat->kswapd_max_order;
 		}
-		finish_wait(&pgdat->kswapd_wait, &wait);
 
 		ret = try_to_freeze();
 		if (kthread_should_stop())