[PATCH] sched/fair: Sync task util before slow-path wakeup

From: Brendan Jackman
Date: Wed Aug 02 2017 - 09:10:19 EST

We use task_util in find_idlest_group via capacity_spare_wake. This
task_util is updated in wake_cap. However wake_cap is not the only
reason for ending up in find_idlest_group - we could have been sent
there by wake_wide. So explicitly sync the task util with prev_cpu
when we are about to head to find_idlest_group.

We could simply do this at the beginning of
select_task_rq_fair (i.e. irrespective of whether we're heading to
select_idle_sibling or find_idlest_group & co), but I didn't want to
slow down the select_idle_sibling path more than necessary.

Don't do this during fork balancing, we won't need the task_util and
we'd just clobber the last_update_time, which is supposed to be 0.

Signed-off-by: Brendan Jackman <brendan.jackman@xxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
kernel/sched/fair.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c95880e216f6..62869ff252b4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5913,6 +5913,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
new_cpu = cpu;

+ if (sd && !(sd_flag & SD_BALANCE_FORK))
+ /*
+ * We're going to need the task's util for capacity_spare_wake
+ * in select_idlest_group. Sync it up to prev_cpu's
+ * last_update_time.
+ */
+ sync_entity_load_avg(&p->se);
if (!sd) {
if (sd_flag & SD_BALANCE_WAKE) /* XXX always ? */