linux-kernel - [PATCH 1/3] sched: select_task_rq() should check cpu_active() like select_fallback

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Date:	Sat, 10 Oct 2015 20:53:09 +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>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] sched: select_task_rq() should check cpu_active() like
	select_fallback_rq()

I do not understand the cpu_active() check in select_fallback_rq().
x86 doesn't need it, and the recent commit dd9d3843755d "sched: Fix
cpu_active_mask/cpu_online_mask race" documents the fact that on any
architecture we can ignore !active starting from CPU_ONLINE stage.

But any possible reason why do we need this check in "fallback" must
equally apply to select_task_rq().

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/sched/core.c |   15 ++++++++-------
 1 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5fe9086..a2ef0cf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1297,6 +1297,11 @@ void kick_process(struct task_struct *p)
 }
 EXPORT_SYMBOL_GPL(kick_process);
 
+static inline bool cpu_allowed(int cpu)
+{
+	return cpu_online(cpu) && cpu_active(cpu);
+}
+
 /*
  * ->cpus_allowed is protected by both rq->lock and p->pi_lock
  */
@@ -1317,9 +1322,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 
 		/* Look for allowed, online CPU in same node. */
 		for_each_cpu(dest_cpu, nodemask) {
-			if (!cpu_online(dest_cpu))
-				continue;
-			if (!cpu_active(dest_cpu))
+			if (!cpu_allowed(dest_cpu))
 				continue;
 			if (cpumask_test_cpu(dest_cpu, tsk_cpus_allowed(p)))
 				return dest_cpu;
@@ -1329,9 +1332,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 	for (;;) {
 		/* Any allowed, online CPU? */
 		for_each_cpu(dest_cpu, tsk_cpus_allowed(p)) {
-			if (!cpu_online(dest_cpu))
-				continue;
-			if (!cpu_active(dest_cpu))
+			if (!cpu_allowed(dest_cpu))
 				continue;
 			goto out;
 		}
@@ -1390,7 +1391,7 @@ int select_task_rq(struct task_struct *p, int cpu, int sd_flags, int wake_flags)
 	 *   not worry about this generic constraint ]
 	 */
 	if (unlikely(!cpumask_test_cpu(cpu, tsk_cpus_allowed(p)) ||
-		     !cpu_online(cpu)))
+		     !cpu_allowed(cpu)))
 		cpu = select_fallback_rq(task_cpu(p), p);
 
 	return cpu;
-- 
1.5.5.1

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