linux-kernel - [PATCH 4/6] sched/fair: Do not migrate due to a sync wakeup on exit

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Message-Id: <20180213133730.24064-5-mgorman@techsingularity.net>
Date:   Tue, 13 Feb 2018 13:37:28 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: [PATCH 4/6] sched/fair: Do not migrate due to a sync wakeup on exit

From: Peter Zijlstra <peterz@...radead.org>

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: Mel Gorman <mgorman@...hsingularity.net>
---
 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 28c8d9c91955..e5bbcbefd01b 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);
-- 
2.15.1