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

[Peter Zijlstra]

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.