Re: rdtsc strangeness on upstream kernel

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 07/19/2010 04:19 PM, Dan Magenheimer wrote:
> In trying to understand some strange behavior I was
> seeing on a RHEL6b2 guest, I ran across an interesting
> anomaly, and it seems to be true on different upstream
> (pvops) kernels.  At first I thought it was a result
> of the xen_sched_clock->xen_clocksource_read patch
> you recently posted, but after some testing this
> appears to be unrelated.
>
> The number of rdtsc/second goes up dramatically when
> there is CPU-intensive load on an upstream kernel!
>   

Are you looking at rdtsc emulation traps when running a PV domain?

> I know we both observed some cases where rdtsc/sec
> was very high, but I don't think we ever were able to
> reproduce this consistently.
>   

It would be interesting to compare that to the context switch rate (cs
column in vmstat output) to see if they correlate.

Also, how does it relate to timer interrupts?

> First, this is a single vcpu, 64-bit 2.6.32 (RHEL6b2)
> kernel.
>
> I am observing ~300 rdtsc/sec on an idle VM.  When
> I run a load of:
>
> main() {while(1);}
>
> I am observing about 10000 rdtsc/sec!!
>
> This is a CONFIG_HZ_1000=y kernel, so I would
> expect 1000 rdtsc/sec, or maybe 2000 rdtsc/sec,
> but 10000 makes me wonder if there is some hidden
> bug.
>   

Do you have preemption running?  And why HZ=1000?

Thanks,
    J