On Fri, 2009-10-09 at 08:35 +0200, Mike Galbraith wrote:
>
> sched: update the clock of runqueue select_task_rq() selected.
>
> In try_to_wake_up(), we update the runqueue clock, but select_task_rq()
> may select a different runqueue than the one we updated, leaving the new
> runqueue's clock stale for a bit.
>
> This patch cures occasional huge latencies reported by latencytop when
> coming out of idle on a mostly idle NO_HZ box.
>
> Signed-off-by: Mike Galbraith <efault@xxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxx>
> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Awesome catch Mike, seems I goofed up in my recent ttwu changes.
Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
>
> ---
> kernel/sched.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -2311,7 +2311,7 @@ static int try_to_wake_up(struct task_st
> {
> int cpu, orig_cpu, this_cpu, success = 0;
> unsigned long flags;
> - struct rq *rq;
> + struct rq *rq, *orig_rq;
>
> if (!sched_feat(SYNC_WAKEUPS))
> wake_flags &= ~WF_SYNC;
> @@ -2319,7 +2319,7 @@ static int try_to_wake_up(struct task_st
> this_cpu = get_cpu();
>
> smp_wmb();
> - rq = task_rq_lock(p, &flags);
> + rq = orig_rq = task_rq_lock(p, &flags);
> update_rq_clock(rq);
> if (!(p->state & state))
> goto out;
> @@ -2350,6 +2350,10 @@ static int try_to_wake_up(struct task_st
> set_task_cpu(p, cpu);
>
> rq = task_rq_lock(p, &flags);
> +
> + if (rq != orig_rq)
> + update_rq_clock(rq);
> +
> WARN_ON(p->state != TASK_WAKING);
> cpu = task_cpu(p);
>
>
>
>
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