Re: [RFC PATCH 2/2] x86/perf/amd: Resolve NMI latency issues when multiple PMCs are active

Peter Zijlstra
2019-03-15T15:12:01Z

On Fri, Mar 15, 2019 at 02:44:32PM +0000, Lendacky, Thomas wrote:

> >> @@ -689,6 +731,7 @@ static __initconst const struct x86_pmu amd_pmu = {
> >>
> >> .amd_nb_constraints = 1,
> >> .wait_on_overflow = amd_pmu_wait_on_overflow,
> >> + .mitigate_nmi_latency = amd_pmu_mitigate_nmi_latency,
> >> };
> >
> > Again, you could just do amd_pmu_handle_irq() and avoid an extra
> > callback.
>
> This is where there would be a bunch of code duplication where I thought
> adding the callback at the end would be better. But if it's best to add
> an AMD handle_irq callback I can do that. I'm easy, let me know if you'd
> prefer that.

Hmm, the thing that avoids you directly using x86_pmu_handle_irq() is
that added active count, but is that not the same as the POPCNT of
cpuc->active_mask?

Is the latency of POPCNT so bad that we need avoid it?

That is, I was thinking of something like:

int amd_pmu_handle_irq(struct pt_regs *regs)
{
struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
int active = hweight_long(cpuc->active_mask);
int handled = x86_pmu_handle_irq(regs);

+ if (active <= 1) {
this_cpu_write(perf_nmi_counter, 0);
+ return handled;
}
+
+ /*
+ * If a counter was handled, record the number of possible remaining
+ * NMIs that can occur.
+ */
+ if (handled) {
+ this_cpu_write(perf_nmi_counter,
+ min_t(unsigned int, 2, active - 1));
+
+ return handled;
+ }
+
+ if (!this_cpu_read(perf_nmi_counter))
+ return NMI_DONE;
+
+ this_cpu_dec(perf_nmi_counter);
+
+ return NMI_HANDLED;
}

> > Anyway, we already had code to deal with spurious NMIs from AMD; see
> > commit:
> >
> > 63e6be6d98e1 ("perf, x86: Catch spurious interrupts after disabling counters")
> >
> > And that looks to be doing something very much the same. Why then do you
> > still need this on top?
>
> This can happen while perf is handling normal counter overflow as opposed
> to covering the disabling of the counter case. When multiple counters
> overflow at roughly the same time, but the NMI doesn't arrive in time to
> get collapsed into a pending NMI, the back-to-back support in
> do_default_nmi() doesn't kick in.
>
> Hmmm... I wonder if the wait on overflow in the disable_all() function
> would eliminate the need for 63e6be6d98e1. That would take a more testing
> on some older hardware to verify. That's something I can look into
> separate from this series.

Yes please, or at least better document the reason for their separate
existence. It's all turning into a bit of magic it seems.