[patch] mm: adjust kswapd nice level for high priority page allocators

[David Rientjes <rientjes@google.com>]

From: Con Kolivas <kernel@kolivas.org>

When kswapd is awoken due to reclaim by a running task, set the priority
of kswapd to that of the task allocating pages thus making memory reclaim
cpu activity affected by nice level.

[rientjes@google.com: refactor for current]
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Con Kolivas <kernel@kolivas.org>
Signed-off-by: David Rientjes <rientjes@google.com>
---
 mm/vmscan.c |   33 ++++++++++++++++++++++++++++++++-
 1 files changed, 32 insertions(+), 1 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1658,6 +1658,33 @@ static void shrink_zone(int priority, struct zone *zone,
 }
 
 /*
+ * Helper functions to adjust nice level of kswapd, based on the priority of
+ * the task allocating pages. If it is already higher priority we do not
+ * demote its nice level since it is still working on behalf of a higher
+ * priority task. With kernel threads we leave it at nice 0.
+ *
+ * We don't ever run kswapd real time, so if a real time task calls kswapd we
+ * set it to highest SCHED_NORMAL priority.
+ */
+static int effective_sc_prio(struct task_struct *p)
+{
+	if (likely(p->mm)) {
+		if (rt_task(p))
+			return -20;
+		return task_nice(p);
+	}
+	return 0;
+}
+
+static void set_kswapd_nice(struct task_struct *kswapd, int active)
+{
+	long nice = effective_sc_prio(current);
+
+	if (task_nice(kswapd) > nice || !active)
+		set_user_nice(kswapd, nice);
+}
+
+/*
  * This is the direct reclaim path, for page-allocating processes.  We only
  * try to reclaim pages from zones which will satisfy the caller's allocation
  * request.
@@ -2257,6 +2284,7 @@ static int kswapd(void *p)
 				}
 			}
 
+			set_user_nice(tsk, 0);
 			order = pgdat->kswapd_max_order;
 		}
 		finish_wait(&pgdat->kswapd_wait, &wait);
@@ -2281,6 +2309,7 @@ static int kswapd(void *p)
 void wakeup_kswapd(struct zone *zone, int order)
 {
 	pg_data_t *pgdat;
+	int active;
 
 	if (!populated_zone(zone))
 		return;
@@ -2292,7 +2321,9 @@ void wakeup_kswapd(struct zone *zone, int order)
 		pgdat->kswapd_max_order = order;
 	if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
 		return;
-	if (!waitqueue_active(&pgdat->kswapd_wait))
+	active = waitqueue_active(&pgdat->kswapd_wait);
+	set_kswapd_nice(pgdat->kswapd, active);
+	if (!active)
 		return;
 	wake_up_interruptible(&pgdat->kswapd_wait);
 }

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