linux-kernel - Re: CFS and suspend2: hang in atomic copy (was: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS])

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Date:	Wed, 18 Apr 2007 18:46:21 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christian Hesse <mail@...thworm.de>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	suspend2-devel@...ts.suspend2.net
Subject: Re: CFS and suspend2: hang in atomic copy (was: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS])


* Christian Hesse <mail@...thworm.de> wrote:

> Hi Ingo and all,
> 
> On Friday 13 April 2007, Ingo Molnar wrote:
> > as usual, any sort of feedback, bugreports, fixes and suggestions are
> > more than welcome,
> 
> I just gave CFS a try on my system. From a user's point of view it 
> looks good so far. Thanks for your work.

you are welcome!

> However I found a problem: When trying to suspend a system patched 
> with suspend2 2.2.9.11 it hangs with "doing atomic copy". Pressing the 
> ESC key results in a message that it tries to abort suspend, but then 
> still hangs.

i took a quick look at suspend2 and it makes some use of yield(). 
There's a bug in CFS's yield code, i've attached a patch that should fix 
it, does it make any difference to the hang?

	Ingo

Index: linux/kernel/sched_fair.c
===================================================================
--- linux.orig/kernel/sched_fair.c
+++ linux/kernel/sched_fair.c
@@ -264,15 +264,26 @@ static void dequeue_task_fair(struct rq 
 
 /*
  * sched_yield() support is very simple via the rbtree, we just
- * dequeue and enqueue the task, which causes the task to
- * roundrobin to the end of the tree:
+ * dequeue the task and move it to the rightmost position, which
+ * causes the task to roundrobin to the end of the tree.
  */
 static void requeue_task_fair(struct rq *rq, struct task_struct *p)
 {
 	dequeue_task_fair(rq, p);
 	p->on_rq = 0;
-	enqueue_task_fair(rq, p);
+	/*
+	 * Temporarily insert at the last position of the tree:
+	 */
+	p->fair_key = LLONG_MAX;
+	__enqueue_task_fair(rq, p);
 	p->on_rq = 1;
+
+	/*
+	 * Update the key to the real value, so that when all other
+	 * tasks from before the rightmost position have executed,
+	 * this task is picked up again:
+	 */
+	p->fair_key = rq->fair_clock - p->wait_runtime + p->nice_offset;
 }
 
 /*
@@ -380,7 +391,10 @@ static void task_tick_fair(struct rq *rq
 	 * Dequeue and enqueue the task to update its
 	 * position within the tree:
 	 */
-	requeue_task_fair(rq, curr);
+	dequeue_task_fair(rq, curr);
+	curr->on_rq = 0;
+	enqueue_task_fair(rq, curr);
+	curr->on_rq = 1;
 
 	/*
 	 * Reschedule if another task tops the current one.
-
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