On Fri, Oct 11, 2019 at 12:28:48PM +0100, Marc Zyngier wrote:
> On Tue, 8 Oct 2019 23:42:22 +0100
> Andrew Murray <andrew.murray@xxxxxxx> 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 <maz@xxxxxxxxxx>
> > > ---
> > > 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
Indeed.
>
> 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.
>
Yeah, I probably need to spend more time understanding this...
> > 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.
That makes sense. That also provides a justification for stopping and
starting the PMU.
>
> > What about ARM 32 bit support for this?
>
> What about it? 32bit KVM/arm doesn't support the PMU at all.
Thanks for the clarification.
Andrew Murray
> 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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