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

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Date:	Fri, 3 Jun 2011 14:49:38 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Borislav Petkov <bp@...64.org>, Borislav Petkov <bp@...en8.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"markus@...ppelsdorf.de" <markus@...ppelsdorf.de>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:sched/urgent] sched: Fix cross-cpu clock sync on remote wakeups

On Thu, Jun 2, 2011 at 11:48 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Thu, 2011-06-02 at 22:23 +0800, Yong Zhang wrote:
>> On Thu, Jun 02, 2011 at 03:04:26PM +0200, Peter Zijlstra wrote:
>> > On Thu, 2011-06-02 at 15:52 +0800, Yong Zhang wrote:
>> > > In sched_clock_local(), clock is calculated around ->tick_gtod even if
>> > > that ->tick_gtod is stale for long time because we stays in idle state.
>> > > You know ->tick_gtod is only updated in sched_clock_tick();
>> >
>> > (well, no, there's idle callbacks as you said below)
>> >
>> > > IOW, when a cpu goes out of idle, sched_clock_tick() is called from
>> > > tick_nohz_stop_idle() which is later than interrupt.
>> >
>> > Gah, that would be awefull and mean wakeups from interrupts were already
>> > borken. /me goes look at code.
>> >
>> > irq_enter() -> tick_check_idle() -> tick_check_nohz() ->
>> > tick_nohz_stop_idle() -> sched_clock_idle_wakeup_event()
>> >
>> > should update the thing before we run any isrs, right?
>>
>> Hmmm, you are right.
>>
>> But smp_reschedule_interrupt() doesn't call irq_enter()/irq_exit(),
>> is that correct?
>
> Crap.. you're right.
> And I bet other archs don't do that either.

Most of them ;)
I only notice sparc32 do that. Maybe there have more,
but I didn't check it very carefully.

> 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).
>
> We could of course add it in sched.c since the logic recurses just
> fine.. its not pretty though.. :/

Yeah, IMHO it's suitable here and my test looks good.

Reviewed-and-Tested-by: Yong Zhang <yong.zhang0@...il.com>

BTW, sched_ipi() and sched_ttwu_pending() could share a piece of
code now. And we place irq_enter()/irq_exit() in sched_ipi() because
it's the only function we could call, thus account_system_vtime() could
get the almost exact time value. IOW we should pay some attention on
the future change of smp_reschedule_interrupt().

Thanks,
Yong

>
> 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)
>
>
>



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