Subject: Re: [PATCH v2 5/5] KVM: arm64: pmu: Reset sample period on overflow handling

From: Marc Zyngier <[email-masked]>
Date: Fri, 11 Oct 2019 12:28:48 +0100

On Tue, 8 Oct 2019 23:42:22 +0100
Andrew Murray <[email-masked]> wrote:

> On Tue, Oct 08, 2019 at 05:01:28PM +0100, Marc Zyngier wrote:
> > The PMU emulation code uses the perf event sample period to trigger
> > the overflow detection. This works fine  for the *first* overflow
> > handling, but results in a huge number of interrupts on the host,
> > unrelated to the number of interrupts handled in the guest (a x20
> > factor is pretty common for the cycle counter). On a slow system
> > (such as a SW model), this can result in the guest only making
> > forward progress at a glacial pace.
> > 
> > It turns out that the clue is in the name. The sample period is
> > exactly that: a period. And once the an overflow has occured,
> > the following period should be the full width of the associated
> > counter, instead of whatever the guest had initially programed.
> > 
> > Reset the sample period to the architected value in the overflow
> > handler, which now results in a number of host interrupts that is
> > much closer to the number of interrupts in the guest.
> > 
> > Fixes: b02386eb7dac ("arm64: KVM: Add PMU overflow interrupt routing")
> > Signed-off-by: Marc Zyngier <[email-masked]>
> > ---
> >  virt/kvm/arm/pmu.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> > 
> > diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
> > index 25a483a04beb..8b524d74c68a 100644
> > --- a/virt/kvm/arm/pmu.c
> > +++ b/virt/kvm/arm/pmu.c
> > @@ -442,6 +442,20 @@ static void kvm_pmu_perf_overflow(struct perf_event *perf_event,
> >  	struct kvm_pmc *pmc = perf_event->overflow_handler_context;
> >  	struct kvm_vcpu *vcpu = kvm_pmc_to_vcpu(pmc);
> >  	int idx = pmc->idx;
> > +	u64 period;
> > +
> > +	/*
> > +	 * Reset the sample period to the architectural limit,
> > +	 * i.e. the point where the counter overflows.
> > +	 */
> > +	period = -(local64_read(&pmc->perf_event->count));
> > +
> > +	if (!kvm_pmu_idx_is_64bit(vcpu, pmc->idx))
> > +		period &= GENMASK(31, 0);
> > +
> > +	local64_set(&pmc->perf_event->hw.period_left, 0);
> > +	pmc->perf_event->attr.sample_period = period;
> > +	pmc->perf_event->hw.sample_period = period;  
> 
> I believe that above, you are reducing the period by the amount period_left
> would have been - they cancel each other out.

That's not what I see happening, having put some traces:

 kvm_pmu_perf_overflow: count = 308 left = 129
 kvm_pmu_perf_overflow: count = 409 left = 47
 kvm_pmu_perf_overflow: count = 585 left = 223
 kvm_pmu_perf_overflow: count = 775 left = 413
 kvm_pmu_perf_overflow: count = 1368 left = 986
 kvm_pmu_perf_overflow: count = 2086 left = 1716
 kvm_pmu_perf_overflow: count = 958 left = 584
 kvm_pmu_perf_overflow: count = 1907 left = 1551
 kvm_pmu_perf_overflow: count = 7292 left = 6932

although I've now moved the stop/start calls inside the overflow
handler so that I don't have to mess with the PMU backend.

> Given that kvm_pmu_perf_overflow is now always called between a
> cpu_pmu->pmu.stop and a cpu_pmu->pmu.start, it means armpmu_event_update
> has been called prior to this function, and armpmu_event_set_period will
> be called after...
> 
> Therefore, I think the above could be reduced to:
> 
> +	/*
> +	 * Reset the sample period to the architectural limit,
> +	 * i.e. the point where the counter overflows.
> +	 */
> +	u64 period = GENMASK(63, 0);
> +	if (!kvm_pmu_idx_is_64bit(vcpu, pmc->idx))
> +		period = GENMASK(31, 0);
> +
> +	pmc->perf_event->attr.sample_period = period;
> +	pmc->perf_event->hw.sample_period = period;
> 
> This is because armpmu_event_set_period takes into account the overflow
> and the counter wrapping via the "if (unlikely(left <= 0)) {" block.

I think that's an oversimplification. As shown above, the counter has
moved forward, and there is a delta to be accounted for.

> Though this code confuses me easily, so I may be talking rubbish.

Same here! ;-)

> 
> >  
> >  	__vcpu_sys_reg(vcpu, PMOVSSET_EL0) |= BIT(idx);
> >  
> > @@ -557,6 +571,7 @@ static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 select_idx)
> >  	attr.exclude_host = 1; /* Don't count host events */
> >  	attr.config = (pmc->idx == ARMV8_PMU_CYCLE_IDX) ?
> >  		ARMV8_PMUV3_PERFCTR_CPU_CYCLES : eventsel;
> > +	attr.config1 = PERF_ATTR_CFG1_RELOAD_EVENT;  
> 
> I'm not sure that this flag, or patch 4 is really needed. As the perf
> events created by KVM are pinned to the task and exclude_(host,hv) are set -
> I think the perf event is not active at this point. Therefore if you change
> the sample period, you can wait until the perf event gets scheduled back in
> (when you return to the guest) where it's call to pmu.start will result in
> armpmu_event_set_period being called. In other words the pmu.start and
> pmu.stop you add in patch 4 is effectively being done for you by perf when
> the KVM task is switched out.
> 
> I'd be interested to see if the following works:
> 
> +	WARN_ON(pmc->perf_event->state == PERF_EVENT_STATE_ACTIVE)
> +
> +	/*
> +	 * Reset the sample period to the architectural limit,
> +	 * i.e. the point where the counter overflows.
> +	 */
> +	u64 period = GENMASK(63, 0);
> +	if (!kvm_pmu_idx_is_64bit(vcpu, pmc->idx))
> +		period = GENMASK(31, 0);
> +
> +	pmc->perf_event->attr.sample_period = period;
> +	pmc->perf_event->hw.sample_period = period;
> 
> >  
> >  	counter = kvm_pmu_get_pair_counter_value(vcpu, pmc);
> >    

The warning fires, which is expected: for event to be inactive, you
need to have the vcpu being scheduled out. When the PMU interrupt
fires, it is bound to preempt the vcpu itself, and the event is of
course still active.

> What about ARM 32 bit support for this?

What about it? 32bit KVM/arm doesn't support the PMU at all. A 32bit
guest on a 64bit host could use the PMU just fine (it is just that
32bit Linux doesn't have a PMUv3 driver -- I had patches for that, but
they never made it upstream).

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

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