linux-kernel - [PATCH 3/6] move_task_off_dead

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Message-ID: <20100315091014.GA9138@redhat.com>
Date:	Mon, 15 Mar 2010 10:10:14 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>
Cc:	Ben Blum <bblum@...gle.com>, Jiri Slaby <jirislaby@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Miao Xie <miaox@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 3/6] move_task_off_dead_cpu: remove retry logic

The previous patch preserved the retry logic, but it looks unneeded.

__migrate_task() can only fail if we raced with migration after we dropped
the lock, but in this case the caller of set_cpus_allowed/etc must initiate
migration itself if ->on_rq == T.

We already fixed p->cpus_allowed, the changes in active/online masks must
be visible to racer, it should migrate the task to online cpu correctly.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---

 kernel/sched.c |   13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

--- 34-rc1/kernel/sched.c~2_XXX_MTODC_DONT_RETRY	2010-03-15 09:40:31.000000000 +0100
+++ 34-rc1/kernel/sched.c	2010-03-15 09:40:44.000000000 +0100
@@ -5517,7 +5517,7 @@ static void move_task_off_dead_cpu(int d
 	struct rq *rq = cpu_rq(dead_cpu);
 	int needs_cpu, dest_cpu;
 	unsigned long flags;
-again:
+
 	local_irq_save(flags);
 
 	raw_spin_lock(&rq->lock);
@@ -5525,14 +5525,13 @@ again:
 	if (needs_cpu)
 		dest_cpu = select_fallback_rq(dead_cpu, p);
 	raw_spin_unlock(&rq->lock);
-
-	/* It can have affinity changed while we were choosing. */
+	/*
+	 * It can only fail if we race with set_cpus_allowed(),
+	 * in the racer should migrate the task anyway.
+	 */
 	if (needs_cpu)
-		needs_cpu = !__migrate_task(p, dead_cpu, dest_cpu);
+		__migrate_task(p, dead_cpu, dest_cpu);
 	local_irq_restore(flags);
-
-	if (unlikely(needs_cpu))
-		goto again;
 }
 
 /*

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