Re: [2.6.16-rc6 patch] smp fix for recalc_task_prio()

[Mike Galbraith <efault@gmx.de>]

On Sat, 2006-03-18 at 05:37 +0100, Mike Galbraith wrote:
> Greetings,
> 
> The patchlet below fixes an smp buglet where a task goes to sleep on one
> cpu and wakes up on another, causing recalc_task_prio() to be called
> with now < p->timestamp.
> 
> If this isn't 2.6.16 material, and I can respin against mm.

Sigh.  Forgot sob line.

	-Mike

Signed-off-by: Mike Galbraith <efault@gmx.de>

--- linux-2.6.16-rc6/kernel/sched.c.org	2006-03-17 14:48:35.000000000 +0100
+++ linux-2.6.16-rc6/kernel/sched.c	2006-03-18 05:22:21.000000000 +0100
@@ -685,6 +685,16 @@
 	unsigned long long __sleep_time = now - p->timestamp;
 	unsigned long sleep_time;
 
+	/*
+	 * On SMP systems, a task can go to sleep on one CPU and
+	 * wake up on another.  When this happens, the timestamp
+	 * is rounded to the nearest tick, which can lead to now
+	 * being less than p->timestamp for short sleeps. Ignore
+	 * these, they're insignificant.
+	 */
+	if (unlikely(now < p->timestamp))
+		__sleep_time = 0ULL;
+
 	if (unlikely(p->policy == SCHED_BATCH))
 		sleep_time = 0;
 	else {


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