linux-kernel - clock drift in set_task

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Message-ID: <AANLkTinNr28LM6NfsmbbmmV28mCzUugTTSI_Bq6N9-8C@mail.gmail.com>
Date:	Wed, 21 Jul 2010 17:10:26 +0530
From:	Jack Daniel <wanders.thirst@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: clock drift in set_task_cpu()

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 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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