linux-kernel - [PATCH 01/40] sched: consult online mask instead of active in select_fallback

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Date:	Mon, 18 Jan 2010 09:57:13 +0900
From:	Tejun Heo <tj@...nel.org>
To:	torvalds@...ux-foundation.org, mingo@...e.hu, peterz@...radead.org,
	awalls@...ix.net, linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
	johannes@...solutions.net, andi@...stfloor.org
Cc:	Tejun Heo <tj@...nel.org>
Subject: [PATCH 01/40] sched: consult online mask instead of active in select_fallback_rq()

If called after sched_class chooses a CPU which isn't in a task's
cpus_allowed mask, select_fallback_rq() can end up migrating a task
which is bound to a !active but online cpu to an active cpu.  This is
dangerous because active is cleared before CPU_DOWN_PREPARE is called
and subsystems expect affinities of kthreads and other tasks to be
maintained till their CPU_DOWN_PREPARE callbacks are complete.

Consult cpu_online_mask instead.

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...e.hu>
---
 kernel/sched.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index c535cc4..09d97e3 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2288,12 +2288,12 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 	const struct cpumask *nodemask = cpumask_of_node(cpu_to_node(cpu));
 
 	/* Look for allowed, online CPU in same node. */
-	for_each_cpu_and(dest_cpu, nodemask, cpu_active_mask)
+	for_each_cpu_and(dest_cpu, nodemask, cpu_online_mask)
 		if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed))
 			return dest_cpu;
 
 	/* Any allowed, online CPU? */
-	dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_active_mask);
+	dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_online_mask);
 	if (dest_cpu < nr_cpu_ids)
 		return dest_cpu;
 
@@ -2302,6 +2302,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 		rcu_read_lock();
 		cpuset_cpus_allowed_locked(p, &p->cpus_allowed);
 		rcu_read_unlock();
+		/* breaking affinity, consider active mask instead */
 		dest_cpu = cpumask_any_and(cpu_active_mask, &p->cpus_allowed);
 
 		/*
-- 
1.6.4.2

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