Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics

Peter Zijlstra
2019-08-28T15:31:46Z

On Mon, Aug 26, 2019 at 07:47:35AM -0700, kan.liang@linux.intel.com wrote:
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 54534ff00940..1ae23db5c2d7 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -76,6 +76,8 @@ u64 x86_perf_event_update(struct perf_event *event)
> if (idx == INTEL_PMC_IDX_FIXED_BTS)
> return 0;
>
> + if (is_topdown_count(event) && x86_pmu.update_topdown_event)
> + return x86_pmu.update_topdown_event(event);

If is_topdown_count() is true; it had better bloody well have that
function. But I really hate this.

> /*
> * Careful: an NMI might modify the previous event value.
> *
> @@ -1003,6 +1005,10 @@ static int collect_events(struct cpu_hw_events *cpuc, struct perf_event *leader,
>
> max_count = x86_pmu.num_counters + x86_pmu.num_counters_fixed;
>
> + /* There are 4 TopDown metrics events. */
> + if (x86_pmu.intel_cap.perf_metrics)
> + max_count += 4;

I'm thinking this is wrong.. this unconditionally allows collecting 4
extra events on every part that has this metrics crud on.

> /* current number of events already accepted */
> n = cpuc->n_events;
>
> @@ -1184,6 +1190,10 @@ int x86_perf_event_set_period(struct perf_event *event)
> if (idx == INTEL_PMC_IDX_FIXED_BTS)
> return 0;
>
> + if (unlikely(is_topdown_count(event)) &&
> + x86_pmu.set_topdown_event_period)
> + return x86_pmu.set_topdown_event_period(event);

Same as with the other method; and I similarly hates it.

> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index f4d6335a18e2..616313d7f3d7 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c

> +static int icl_set_topdown_event_period(struct perf_event *event)
> +{
> + struct hw_perf_event *hwc = &event->hw;
> + s64 left = local64_read(&hwc->period_left);
> +
> + /*
> + * Clear PERF_METRICS and Fixed counter 3 in initialization.
> + * After that, both MSRs will be cleared for each read.
> + * Don't need to clear them again.
> + */
> + if (left == x86_pmu.max_period) {
> + wrmsrl(MSR_CORE_PERF_FIXED_CTR3, 0);
> + wrmsrl(MSR_PERF_METRICS, 0);
> + local64_set(&hwc->period_left, 0);
> + }

This really doesn't make sense to me; if you set FIXED_CTR3 := 0, you'll
never trigger the overflow there; this then seems to suggest the actual
counter value is irrelevant. Therefore you don't actually need this.

> +
> + perf_event_update_userpage(event);
> +
> + return 0;
> +}
> +
> +static u64 icl_get_metrics_event_value(u64 metric, u64 slots, int idx)
> +{
> + u32 val;
> +
> + /*
> + * The metric is reported as an 8bit integer percentage

s/percentage/fraction/

percentage is 1/100, 8bit is 256.

> + * suming up to 0xff.
> + * slots-in-metric = (Metric / 0xff) * slots
> + */
> + val = (metric >> ((idx - INTEL_PMC_IDX_FIXED_METRIC_BASE) * 8)) & 0xff;
> + return mul_u64_u32_div(slots, val, 0xff);

This really utterly blows.. I'd have wasted range to be able to do a
power-of-two fraction here. That is use 8 bits with a range [0,128].

But also; x86_64 seems to lack a sane implementation of that function,
and it currently compiles into utter crap (it can be 2 instructions).

> +}

> +/*
> + * Update all active Topdown events.
> + * PMU has to be disabled before calling this function.

Forgets to explain why...

> + */