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Subject: Re: [patch] sched: add the other missing clock update to
 migrate_task()
From: Peter Zijlstra <peterz@infradead.org>
To: Mike Galbraith <efault@gmx.de>
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>
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Date: Sun, 22 Nov 2009 14:16:10 +0100
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On Sun, 2009-11-22 at 13:11 +0100, Mike Galbraith wrote:
> sched: add the other missing clock update to migrate_task()
> 
> When calling set_task_cpu(), we must update both runqueue clocks in order
> to get an accurate clock offset.  Add it.
> 
> 
> Signed-off-by: Mike Galbraith <efault@gmx.de>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> LKML-Reference: <new-submission>
> 
> ---
>  kernel/sched.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -2126,6 +2126,7 @@ migrate_task(struct task_struct *p, int
>  	 */
>  	if (!p->se.on_rq && !task_running(rq, p)) {
>  		update_rq_clock(rq);
> +		update_rq_clock(cpu_rq(dest_cpu));
>  		set_task_cpu(p, dest_cpu);
>  		return 0;
>  	}


We should make double_rq_lock() and double_lock_balance() behave
equivalent wrt update_rq_clock().

Current, depending on CONFIG_PREEMPT, double_lock_balance() already
updates both rq clocks.