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

Lendacky, Thomas
2019-03-15T14:44:41Z

On 3/15/19 7:03 AM, Peter Zijlstra wrote:
> On Mon, Mar 11, 2019 at 04:48:51PM +0000, Lendacky, Thomas wrote:
>> @@ -467,6 +470,45 @@ static void amd_pmu_wait_on_overflow(int idx, u64 config)
>> }
>> }
>>
>> +/*
>> + * Because of NMI latency, if multiple PMC counters are active we need to take
>> + * into account that multiple PMC overflows can generate multiple NMIs but be
>> + * handled by a single invocation of the NMI handler (think PMC overflow while
>> + * in the NMI handler). This could result in subsequent unknown NMI messages
>> + * being issued.
>> + *
>> + * Attempt to mitigate this by using the number of active PMCs to determine
>> + * whether to return NMI_HANDLED if the perf NMI handler did not handle/reset
>> + * any PMCs. The per-CPU perf_nmi_counter variable is set to a minimum of one
>> + * less than the number of active PMCs or 2. The value of 2 is used in case the
>> + * NMI does not arrive at the APIC in time to be collapsed into an already
>> + * pending NMI.
>
> LAPIC I really do hope?!

Yes, LAPIC.

>
>> + */
>> +static int amd_pmu_mitigate_nmi_latency(unsigned int active, int handled)
>> +{
>> + /* If multiple counters are not active return original handled count */
>> + if (active <= 1)
>> + return handled;
>
> Should we not reset perf_nmi_counter in this case?

Yes, we should. I'll make that change.

>
>> +
>> + /*
>> + * 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;
>> +}
>> +
>> static struct event_constraint *
>> amd_get_event_constraints(struct cpu_hw_events *cpuc, int idx,
>> struct perf_event *event)
>> @@ -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.

>
> 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.

Thanks,
Tom

>
>