linux-kernel - Re: clock drift in set_task

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Date:	Mon, 6 Sep 2010 12:04:55 +0530
From:	Jack Daniel <wanders.thirst@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>, pjohn@...sta.com
Subject: Re: clock drift in set_task_cpu()

Hi Ingo,

On Mon, Aug 9, 2010 at 8:26 PM, Philby John <pjohn@...sta.com> wrote:
> On Mon, 2010-08-09 at 18:47 +0530, Jack Daniel wrote:
>> On Thu, Aug 5, 2010 at 3:28 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Wed, 2010-07-21 at 17:10 +0530, Jack Daniel wrote:
>> >> On a Xeon 55xx with 8 CPU's, I found out the new_rq->clock value is
>> >> sometimes larger than old_rq->clock and so clock_offset tends to warp
>> >> around leading to incorrect values.
>> >
>> > What values get incorrect, do you observe vruntime funnies or only the
>> > schedstat values?
>>
>> Just the schedstat values, did not observe anything wrong with vruntime.
>>
>> >
>> >>  You have very correctly noted in
>> >> the commit header that all functions that access set_task_cpu() must
>> >> do so after a call to sched_clock_remote(), in this case the function
>> >> is sched_fork(). I validated by adding update_rq_clock(old_rq); into
>> >> set_task_cpu() and that seems to fix the issue.
>> >
>> > Ah, so the problem is that task_fork_fair() does the task placement
>> > without updated rq clocks? In which case I think we should at least do
>> > an update_rq_clock(rq) in there (see the below patch).
>>
>> Yes, this is indeed the problem and your patch seems to fix the issue.
>>
>> >
>> >> But I noticed that
>> >> since CONFIG_HAVE_UNSTABLE_SCHED_CLOCK is already set, if
>> >> (sched_clock_stable)  in sched_clock_cpu() will yield to true and the
>> >> flow never gets to sched_clock_remote() or sched_clock_local().
>> >
>> > sched_clock_stable being true implies the clock is stable across cores
>> > and thus it shouldn't matter. Or are you saying you're seeing it being
>> > set and still have issues?
>> >
>>
>> Please ignore these comments, initial debugging set me on the wrong
>> foot, to suggest that TSC is unstable.
>>
>> > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
>> > index 9910e1b..f816e74 100644
>> > --- a/kernel/sched_fair.c
>> > +++ b/kernel/sched_fair.c
>> > @@ -3751,6 +3751,8 @@ static void task_fork_fair(struct task_struct *p)
>> >
>> >        raw_spin_lock_irqsave(&rq->lock, flags);
>> >
>> > +       update_rq_clock(rq);
>>
>> As you rightly pointed out above, updating the clocks in
>> task_fork_fair() will rightly fix the issue. Can get rid of rest of
>> the update_rq_clock() functions as they (like you said), are expensive
>> and I tested commenting them out.
>
> >From 1bc695bc2ac6c941724953b29f6c18196a474b8f Mon Sep 17 00:00:00 2001
> From: Philby John <pjohn@...sta.com>
> Date: Mon, 9 Aug 2010 18:19:08 +0530
> Subject: [PATCH] sched: ensure rq->clock get sync'ed when migrating tasks
>
> In sched_fork() when we do task placement in ->task_fork_fair()
> ensure we update_rq_clock() so we work with current time. This has
> been noted and verified on an Intel Greencity (Xeon 55xx)
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Signed-off-by: Philby John <pjohn@...sta.com>
> ---
>  kernel/sched_fair.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 806d1b2..48bc31c 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -3751,7 +3751,7 @@ static void task_fork_fair(struct task_struct *p)
>        unsigned long flags;
>
>        raw_spin_lock_irqsave(&rq->lock, flags);
> -
> +       update_rq_clock(rq);
>        if (unlikely(task_cpu(p) != this_cpu))
>                __set_task_cpu(p, this_cpu);
>

Any chance that you will be pulling in this fix ?

Regards,
Jack
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