linux-kernel - [tip:sched/core] stop_machine: Avoid a sleep and wakeup in stop_one

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Message-ID: <tip-bf89a304722f6904009499a31dc68ab9a5c9742e@git.kernel.org>
Date:   Thu, 22 Sep 2016 06:59:24 -0700
From:   tip-bot for Cheng Chao <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, oleg@...hat.com, tglx@...utronix.de,
        mingo@...nel.org, hpa@...or.com, torvalds@...ux-foundation.org,
        cs.os.kernel@...il.com, peterz@...radead.org
Subject: [tip:sched/core] stop_machine: Avoid a sleep and wakeup in
 stop_one_cpu()

Commit-ID:  bf89a304722f6904009499a31dc68ab9a5c9742e
Gitweb:     http://git.kernel.org/tip/bf89a304722f6904009499a31dc68ab9a5c9742e
Author:     Cheng Chao <cs.os.kernel@...il.com>
AuthorDate: Wed, 14 Sep 2016 10:01:50 +0800
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Thu, 22 Sep 2016 14:53:45 +0200

stop_machine: Avoid a sleep and wakeup in stop_one_cpu()

In case @cpu == smp_proccessor_id(), we can avoid a sleep+wakeup
cycle by doing a preemption.

Callers such as sched_exec() can benefit from this change.

Signed-off-by: Cheng Chao <cs.os.kernel@...il.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: akpm@...ux-foundation.org
Cc: chris@...is-wilson.co.uk
Cc: tj@...nel.org
Link: http://lkml.kernel.org/r/1473818510-6779-1-git-send-email-cs.os.kernel@gmail.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/core.c   | 8 ++++++--
 kernel/stop_machine.c | 5 +++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c5f020c..ff4e3c0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1063,8 +1063,12 @@ static int migration_cpu_stop(void *data)
 	 * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because
 	 * we're holding p->pi_lock.
 	 */
-	if (task_rq(p) == rq && task_on_rq_queued(p))
-		rq = __migrate_task(rq, p, arg->dest_cpu);
+	if (task_rq(p) == rq) {
+		if (task_on_rq_queued(p))
+			rq = __migrate_task(rq, p, arg->dest_cpu);
+		else
+			p->wake_cpu = arg->dest_cpu;
+	}
 	raw_spin_unlock(&rq->lock);
 	raw_spin_unlock(&p->pi_lock);
 
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 4a1ca5f..082e71f 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -126,6 +126,11 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
 	cpu_stop_init_done(&done, 1);
 	if (!cpu_stop_queue_work(cpu, &work))
 		return -ENOENT;
+	/*
+	 * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup
+	 * cycle by doing a preemption:
+	 */
+	cond_resched();
 	wait_for_completion(&done.completion);
 	return done.ret;
 }