Re: [tip:sched/urgent] sched: Fix cross-cpu clock sync on remote wakeups

[Borislav Petkov <bp@amd64.org>]

On Thu, Jun 02, 2011 at 11:48:31AM -0400, Peter Zijlstra wrote:
> Crap.. you're right. And I bet other archs don't do that either. With
> NO_HZ you really need irq_enter() for pretty much all interrupts so I
> was assuming the resched IPI had it, but its been special and never
> really needed it. If it would wake an idle cpu the idle loop exit would
> deal with it, if it interrupted userspace the thing was running and
> NO_HZ wasn't relevant.
> 
> Damn. 
> 
> And yes, the only reason I didn't see this on my dev box was because we
> do indeed set that sched_clock_stable thing on wsm. And I never noticed
> on my desktop because firefox/X/etc. consuming heaps of CPU isn't weird
> at all.
> 
> Adding it to all resched int handlers is of course a possibility but
> would slow down the thing, although with the new code, most users are
> now indeed wakeups (excepting weird and wonderful users like KVM).

FWIW, we could set the sched_clock_stable on AMD too - at least on F10h
and later. This will take care of the problem at hand and defer the
issue of slowing down the resched ipi handlers.

I dunno, however, whether we still would need the proper ->tick_gtod
update for correct ttwu accounting regardless of sched_clock_stable on
K8 (unstable TSCs) and maybe even other arches.

> 
> We could of course add it in sched.c since the logic recurses just
> fine.. its not pretty though.. :/
> 
> Thoughts?
> 
> ---
>  kernel/sched.c |   18 +++++++++++++++++-
>  1 files changed, 17 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 2fe98ed..365ed6b 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2554,7 +2554,23 @@ static void sched_ttwu_pending(void)
>  
>  void scheduler_ipi(void)
>  {
> -	sched_ttwu_pending();
> +	struct rq *rq = this_rq();
> +	struct task_struct *list = xchg(&rq->wake_list, NULL);
> +
> +	if (!list)
> +		return;
> +
> +	irq_enter();
> +	raw_spin_lock(&rq->lock);
> +
> +	while (list) {
> +		struct task_struct *p = list;
> +		list = list->wake_entry;
> +		ttwu_do_activate(rq, p, 0);
> +	}
> +
> +	raw_spin_unlock(&rq->lock);
> +	irq_exit();
>  }
>  
>  static void ttwu_queue_remote(struct task_struct *p, int cpu)
> 
> 
> 

-- 
Regards/Gruss,
Boris.

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