linux-kernel - Re: clock drift in set_task

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Message-ID: <AANLkTikIt_DEDKdYB7BuikGlCBEN92YsASCrQ0U-hT1H@mail.gmail.com>
Date:	Thu, 22 Jul 2010 11:04:50 +0530
From:	Jack Daniel <wanders.thirst@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: clock drift in set_task_cpu()

Greetings,

I would be much obliged if anyone can answer my below query. Any
guidance or advice is much appreciated. I believe the problem still
exists in the new kernel versions.

Thanks and regards,
Jack

On Wed, Jul 21, 2010 at 5:10 PM, Jack Daniel <wanders.thirst@...il.com> wrote:
> Hi Peter/Ingo,
>
> I have a query with the kernel code that was changed not too long time
> back in v2.6.33-rc1 commit id 5afcdab706d6002cb02b567ba46e650215e694e8
> [tip:sched/urgent] sched: Remove rq->clock coupling from
> set_task_cpu()
>
> void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> {
> int old_cpu = task_cpu(p);
> struct rq *old_rq = cpu_rq(old_cpu), *new_rq = cpu_rq(new_cpu);
> struct cfs_rq *old_cfsrq = task_cfs_rq(p),
>      *new_cfsrq = cpu_cfs_rq(old_cfsrq, new_cpu);
> u64 clock_offset;
>
> clock_offset = old_rq->clock - new_rq->clock;
> ---
>
> 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. 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. 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().
>
> What do you think is the best way to approach the problem *assuming
> the older kernel*, since I believe the problem still exists? That is
> to reinstate your axiom ".... which should ensure the observed time
> between these two cpus is monotonic"
>
> 1) CONFIG_HAVE_UNSTABLE_SCHED_CLOCK cannot be disabled since it is set
> by default for x86
> 2) Does one create a new function with just this line of pseudo code?
> fix_clock_drift()
> {
> if (cpu != smp_processor_id())
>                clock = sched_clock_remote(scd);
>        else
>                clock = sched_clock_local(scd);
>
>        return clock;
> }
>
> Thanks and regards,
> Jack
>
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