Re: [PATCH v32 0/5]Timestamp synchronization of host - guest tracing session

[Tzvetomir Stoyanov <tz.stoyanov@gmail.com>]

Hi Stefano,

On Fri, Mar 19, 2021 at 7:44 PM Stefano De Venuto
<stefano.devenuto99@gmail.com> wrote:
>
>
>
> On 3/19/21 12:55 PM, Tzvetomir Stoyanov wrote:
> > Hi Stefano,
> >
> > On Fri, Mar 19, 2021 at 12:08 PM Stefano De Venuto
> > <stefano.devenuto99@gmail.com> wrote:
> Hi!
> >> The commands used to record are:
> >>
> >> Host:
> >> # trace-cmd record -C x86-tsc -e kvm:* -e msr:* -A tumbleweed:823 -e
> >> msr:* -C x86-tsc sleep 1
> > The guest trace clock is set automatically as the host, so this
> > command should be enough:
> > # trace-cmd record -C x86-tsc -e kvm:* -e msr:* -A tumbleweed:823 -e
> > msr:*  sleep 1
> >
> >> Guest:
> >> # echo x86-tsc > /sys/kernel/tracing/trace_clock
> > There is no need to set manually the guest clock, it will be
> > overwritten by trace-cmd agent.
> >
> Thanks so much for the proper way to do it, really appreciated.
> >> If necessary, I can provide more info about my setup, or do more tests.
> > Yes, please can you send me both host and guest trace files ?
> Here are the trace files, host and guest respectively:
>
> - http://xenbits.xen.org/people/dariof/tracing-examples/kvm/sync-kvmclock/trace.dat
> - http://xenbits.xen.org/people/dariof/tracing-examples/kvm/sync-kvmclock/trace-tumbleweed.dat
>
> > Also, it will be useful to send me the content of the KVM debug files:
> >     /sys/kernel/debug/kvm/<guest ID>/vcpu<*>/tsc-offset
> The guest has one vcpu (vcpu0) and the content of the file is:
>
> 255647917761327

Looks like there is a scaling between host and guest clocks in your
setup, not just a simple offset. We did not test yet our
implementation with scaling, although both offset and scaling are part
of the calculations. That makes your use case very valuable for us, as
we have an opportunity to test it now. And yes, looks like we have a
bug here.
Please, when you have time, can you repeat again the tracing session
and send again both trace files + the content of the KVM debug files:
     /sys/kernel/debug/kvm/<guest ID>/vcpu0/tsc-offset
     /sys/kernel/debug/kvm/<guest ID>/vcpu0/tsc-scaling-ratio
I'm asking to do a new trace, as most probably these offset and
scaling could be different now.

Thanks!

> >>
> >> Thanks and Regards,
> >>
> >> Stefano
> > Thanks for testing this code!
> >
> Thanks for your time,
>
> Stefano



-- 
Tzvetomir (Ceco) Stoyanov
VMware Open Source Technology Center