Re: [PATCH 2/3] KVM: x86/vPMU: Reuse perf_event to avoid unnecessary pmc_reprogram_counter

[Like Xu]

Hi Peter,

On 2019/10/1 16:22, Peter Zijlstra wrote:
> On Mon, Sep 30, 2019 at 03:22:56PM +0800, Like Xu wrote:
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index 46875bbd0419..74bc5c42b8b5 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -140,6 +140,35 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
> >   	clear_bit(pmc->idx, (unsigned long*)&pmc_to_pmu(pmc)->reprogram_pmi);
> >   }
> >   
> > +static void pmc_pause_counter(struct kvm_pmc *pmc)
> > +{
> > +	if (!pmc->perf_event)
> > +		return;
> > +
> > +	pmc->counter = pmc_read_counter(pmc);
> > +
> > +	perf_event_disable(pmc->perf_event);
> > +
> > +	/* reset count to avoid redundant accumulation */
> > +	local64_set(&pmc->perf_event->count, 0);
>
> Yuck, don't frob in data structures you don't own.

Yes, it's reasonable. Thanks.

> Just like you exported the IOC_PERIOD thing, so too is there a
> IOC_RESET.
>
> Furthermore; wth do you call pmc_read_counter() _before_ doing
> perf_event_disable() ? Doing it the other way around is much cheaper,
> even better, you can use perf_event_count() after disable.

Yes, it's much better and let me apply this.

> > +}
> > +
> > +static bool pmc_resume_counter(struct kvm_pmc *pmc)
> > +{
> > +	if (!pmc->perf_event)
> > +		return false;
> > +
> > +	/* recalibrate sample period and check if it's accepted by perf core */
> > +	if (perf_event_period(pmc->perf_event,
> > +			(-pmc->counter) & pmc_bitmask(pmc)))
> > +		return false;
>
> I'd do the reset here, but then you have 3 function in a row that do
> perf_event_ctx_lock()+perf_event_ctx_unlock(), which is rather
> expensive.

Calling pmc_pause_counter() is not always followed by calling 
pmc_resume_counter(). The former may be called multiple times before the 
later is called, so if we do not reset event->count in the 
pmc_pause_counter(), it will be repeatedly accumulated into pmc->counter 
which is a functional error.

> > +
> > +	/* reuse perf_event to serve as pmc_reprogram_counter() does*/
> > +	perf_event_enable(pmc->perf_event);
> > +	clear_bit(pmc->idx, (unsigned long *)&pmc_to_pmu(pmc)->reprogram_pmi);
> > +	return true;
> > +}