linux-kernel - [tip:sched/core] sched/fair: Do not migrate due to a sync wakeup on exit

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Date:   Wed, 21 Feb 2018 02:29:07 -0800
From:   tip-bot for Peter Zijlstra <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     torvalds@...ux-foundation.org, hpa@...or.com, ggherdovich@...e.cz,
        efault@....de, mgorman@...hsingularity.net, tglx@...utronix.de,
        peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, matt@...eblueprint.co.uk
Subject: [tip:sched/core] sched/fair: Do not migrate due to a sync wakeup on
 exit

Commit-ID:  24d0c1d6e65f635b2c0684d0a42ff6c0674aa0e6
Gitweb:     https://git.kernel.org/tip/24d0c1d6e65f635b2c0684d0a42ff6c0674aa0e6
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Tue, 13 Feb 2018 13:37:28 +0000
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 21 Feb 2018 08:49:42 +0100

sched/fair: Do not migrate due to a sync wakeup on exit

When a task exits, it notifies the parent that it has exited. This is a
sync wakeup and the exiting task may pull the parent towards the wakers
CPU. For simple workloads like using a shell, it was observed that the
shell is pulled across nodes by exiting processes. This is daft as the
parent may be long-lived and properly placed. This patch special cases a
sync wakeup on exit to avoid pulling tasks across nodes. Testing on a range
of workloads and machines showed very little differences in performance
although there was a small 3% boost on some machines running a shellscript
intensive workload (git regression test suite).

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Giovanni Gherdovich <ggherdovich@...e.cz>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Matt Fleming <matt@...eblueprint.co.uk>
Cc: Mike Galbraith <efault@....de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20180213133730.24064-5-mgorman@techsingularity.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a07920f..302dda8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6350,7 +6350,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
 	int cpu = smp_processor_id();
 	int new_cpu = prev_cpu;
 	int want_affine = 0;
-	int sync = wake_flags & WF_SYNC;
+	int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
 
 	if (sd_flag & SD_BALANCE_WAKE) {
 		record_wakee(p);