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

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Date:   Wed, 28 Aug 2019 17:19:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     acme@...nel.org, mingo@...hat.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, jolsa@...nel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com, ak@...ux.intel.com
Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown
 metrics

On Mon, Aug 26, 2019 at 07:47:35AM -0700, kan.liang@...ux.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...

> + */