[RFC][PATCH] sched: Avoid select_idle_sibling() for wake_affine(.sync=true)

[Peter Zijlstra <peterz@infradead.org>]

Subject: sched: Avoid select_idle_sibling() for wake_affine(.sync=true)
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed Sep 25 08:28:39 CEST 2013

When a task is the only running task and does a sync wakeup; avoid
going through select_idle_sibling() as it doesn't know the current CPU
is going to be idle shortly.

Without this two sync wakers will ping-pong between CPUs for no
reason.

Suggested-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
 kernel/sched/fair.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3461,6 +3461,16 @@ select_task_rq_fair(struct task_struct *
 		if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
 			prev_cpu = cpu;
 
+		/*
+		 * Don't bother with select_idle_sibling() in the case of a sync wakeup
+		 * where we know the only running task will soon go away. Going
+		 * through select_idle_sibling will only lead to pointless ping-pong.
+		 */
+		if (sync && prev_cpu == cpu && cpu_rq(cpu)->nr_running == 1) {
+			new_cpu = cpu;
+			goto unlock;
+		}
+
 		new_cpu = select_idle_sibling(p, prev_cpu);
 		goto unlock;
 	}