linux-kernel - [PATCH] sched: wake up task on prev_cpu if not in SD_WAKE

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Date:	Fri, 9 May 2014 01:27:43 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	linux-kernel@...r.kernel.org, morten.rasmussen@....com,
	mingo@...nel.org, george.mccollister@...il.com,
	ktkhai@...allels.com
Subject: [PATCH] sched: wake up task on prev_cpu if not in SD_WAKE_AFFINE
 domain with cpu

On Thu, 08 May 2014 22:20:25 -0400
Rik van Riel <riel@...hat.com> wrote:

> Looks like SD_BALANCE_WAKE is not gotten from the sd flags at
> all, but passed into select_task_rq by try_to_wake_up, as a
> hard coded sd_flags argument.

> Should we do that, if SD_WAKE_BALANCE is not set for any sched domain?

I answered my own question. The sd_flag SD_WAKE_BALANCE simply means
"this is a wakeup of a previously existing task, please place it
properly".

However, it appears that the current code will fall back to the large
loop with select_idlest_group and friends, if prev_cpu and cpu are not
part of the same SD_WAKE_AFFINE sched domain. That is a bug...

---8<---

Subject: sched: wake up task on prev_cpu if not in SD_WAKE_AFFINE domain with cpu

If prev_cpu and cpu are not part of the same SD_WAKE_AFFINE domain,
the current code will fall back to the select_idlest_group CPU selection
mechanism, instead of trying to wake up the task on a CPU near its
previous CPU.

Fix that by always calling select_idle_sibling when doing an SD_BALANCE_WAKE
wakeup.

Signed-off-by: Rik van Riel <riel@...hat.com>
---
 kernel/sched/fair.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7570dd9..5d33fb1b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4452,7 +4452,9 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
 	if (affine_sd) {
 		if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
 			prev_cpu = cpu;
+	}
 
+	if (sd_flag & SD_BALANCE_WAKE) {
 		new_cpu = select_idle_sibling(p, prev_cpu);
 		goto unlock;
 	}

--
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