sched-move_task_off_dead_cpu-Remove-retry-logic.patch

From 88c62d921bbb9b93c431e920e0719473e9bfca42 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Mon, 15 Mar 2010 10:10:14 +0100
Subject: [PATCH] sched: move_task_off_dead_cpu(): Remove retry logic

commit 2d5a7c7241f20ee8e90452a47b773cffbb4c2412 in tip.

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.

[ upstream commit: c1804d547dc098363443667609c272d1e4d15ee8 ]

[PG: warning; this differs from upstream by the critical change:
   -       needs_cpu = (task_cpu(p) == dead_cpu) && (p->state != TASK_WAKING);
   +       needs_cpu = (task_cpu(p) == dead_cpu) && !(p->state & TASK_WAKING);
 which makes a big difference when on RT, TASK_WAKING is not exclusive.]

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100315091014.GA9138@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

diff --git a/kernel/sched.c b/kernel/sched.c
index 30e59b6..dfc2d66 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -7720,7 +7720,7 @@ static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
 	struct rq *rq = cpu_rq(dead_cpu);
 	int needs_cpu, uninitialized_var(dest_cpu);
 	unsigned long flags;
-again:
+
 	local_irq_save(flags);
 
 	raw_spin_lock(&rq->lock);
@@ -7728,14 +7728,13 @@ 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;
 }
 
 /*
-- 
1.7.1.1