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SubjectRe: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples
On Tue, 19 Feb 2013, Thomas Gleixner wrote:
> On Tue, 19 Feb 2013, John Stultz wrote:
> Would be interesting to compare and contrast that. Though you can't do
> that in the kernel as the write hold time of the timekeeper seq is way
> larger than the gtod->seq write hold time. I have a patch series in
> work which makes the timekeeper seq hold time almost as short as that
> of gtod->seq.

As a side note. There is a really interesting corner case
vs. virtualization.


write_seqlock_irqsave(&tk->lock, flags);

Host schedules out VCPU0

Arbitrary delay

Host schedules in VCPU0

Depending on the length of the delay which kept VCPU0 away from
executing and depending on the direction of the ntp update of the
timekeeping variables __vdso_clock_gettime()#2 can observe time going

You can reproduce that by pinning VCPU0 to physical core 0 and VCPU1
to physical core 1. Now remove all load from physical core 1 except
VCPU1 and put massive load on physical core 0 and make sure that the
NTP adjustment lowers the mult factor.

Fun, isn't it ?


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