linux-kernel - [PATCH 3/3] sched: start stopper early

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Date:	Fri, 9 Oct 2015 18:00:54 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	heiko.carstens@...ibm.com, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 3/3] sched: start stopper early

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

Ensure the stopper thread is active 'early', because the load balancer
pretty much assumes that its available. And when 'online && active' the
load-balancer is fully available.

Not only the numa balancing stop_two_cpus() caller relies on it, but
also the self migration stuff does, and at CPU_ONLINE time the cpu
really is 'free' to run anything.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/cpu.c        |    1 -
 kernel/sched/core.c |    5 ++++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index ef52b3b..8a7225a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -464,7 +464,6 @@ static int smpboot_thread_call(struct notifier_block *nfb,
 	switch (action & ~CPU_TASKS_FROZEN) {
 
 	case CPU_ONLINE:
-		stop_machine_unpark(cpu);
 		smpboot_unpark_threads(cpu);
 		break;
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e691052..955c181 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5324,11 +5324,14 @@ static void __cpuinit set_cpu_rq_start_time(void)
 static int sched_cpu_active(struct notifier_block *nfb,
 				      unsigned long action, void *hcpu)
 {
+	int cpu = (long)hcpu;
+
 	switch (action & ~CPU_TASKS_FROZEN) {
 	case CPU_STARTING:
 		set_cpu_rq_start_time();
 		return NOTIFY_OK;
 	case CPU_ONLINE:
+		stop_machine_unpark(cpu);
 		/*
 		 * At this point a starting CPU has marked itself as online via
 		 * set_cpu_online(). But it might not yet have marked itself
@@ -5337,7 +5340,7 @@ static int sched_cpu_active(struct notifier_block *nfb,
 		 * Thus, fall-through and help the starting CPU along.
 		 */
 	case CPU_DOWN_FAILED:
-		set_cpu_active((long)hcpu, true);
+		set_cpu_active(cpu, true);
 		return NOTIFY_OK;
 	default:
 		return NOTIFY_DONE;
-- 
1.5.5.1

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