[PATCH 03/16] sched/fair: Disregard idle task wakee_flips in wake_wide

[Morten Rasmussen]

wake_wide() is based on task wakee_flips of the waker and the wakee to
decide whether an affine wakeup is desirable. On lightly loaded systems
the waker is frequently the idle task (pid=0) which can accumulate a lot
of wakee_flips in that scenario. It makes little sense to prevent affine
wakeups on an idle cpu due to the idle task wakee_flips, so it makes
more sense to ignore them in wake_wide().

cc: Ingo Molnar <mingo@xxxxxxxxxx>
cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Signed-off-by: Morten Rasmussen <morten.rasmussen@xxxxxxx>
---
 kernel/sched/fair.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c49e25a..0fe3020 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5007,6 +5007,10 @@ static int wake_wide(struct task_struct *p)
 	unsigned int slave = p->wakee_flips;
 	int factor = this_cpu_read(sd_llc_size);
 
+	/* Don't let the idle task prevent affine wakeups */
+	if (is_idle_task(current))
+		return 0;
+
 	if (master < slave)
 		swap(master, slave);
 	if (slave < factor || master < slave * factor)
-- 
1.9.1