Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples

[Thomas Gleixner <tglx@linutronix.de>]

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.

  VCPU0						VCPU1

  update_wall_time()
    write_seqlock_irqsave(&tk->lock, flags);
    ....

Host schedules out VCPU0

Arbitrary delay

Host schedules in VCPU0
						__vdso_clock_gettime()#1
    update_vsyscall();
						__vdso_clock_gettime()#2

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
backwards.

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 ?

     tglx