linux-kernel - Re: [PATCH V7 08/14] perf/x86/intel: Generic support for hardware TopDown metrics

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Date:   Fri, 24 Jul 2020 15:19:06 +0200
From:   peterz@...radead.org
To:     kan.liang@...ux.intel.com
Cc:     acme@...hat.com, mingo@...nel.org, linux-kernel@...r.kernel.org,
        jolsa@...nel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com, ak@...ux.intel.com,
        like.xu@...ux.intel.com
Subject: Re: [PATCH V7 08/14] perf/x86/intel: Generic support for hardware
 TopDown metrics

On Thu, Jul 23, 2020 at 10:11:11AM -0700, kan.liang@...ux.intel.com wrote:
> @@ -3375,6 +3428,72 @@ static int intel_pmu_hw_config(struct perf_event *event)
>  	if (event->attr.type != PERF_TYPE_RAW)
>  		return 0;
>  
> +	/*
> +	 * Config Topdown slots and metric events
> +	 *
> +	 * The slots event on Fixed Counter 3 can support sampling,
> +	 * which will be handled normally in x86_perf_event_update().
> +	 *
> +	 * The metric events don't support sampling.
> +	 *
> +	 * For counting, topdown slots and metric events will be
> +	 * handled specially for event update.
> +	 * A flag PERF_X86_EVENT_TOPDOWN is applied for the case.
> +	 */
> +	if (x86_pmu.intel_cap.perf_metrics && is_topdown_event(event)) {
> +		if (is_metric_event(event)) {
> +			struct perf_event *leader = event->group_leader;
> +			struct perf_event *sibling;
> +
> +			/* The metric events don't support sampling. */
> +			if (is_sampling_event(event))
> +				return -EINVAL;
> +
> +			/* The metric events cannot be a group leader. */
> +			if (leader == event)
> +				return -EINVAL;
> +
> +			/*
> +			 * The slots event cannot be the leader of a topdown
> +			 * sample-read group, e.g., {slots, topdown-retiring}:S
> +			 */
> +			if (is_slots_event(leader) && is_sampling_event(leader))
> +				return -EINVAL;

This has nothing to do with sample-read; SLOTS cannot be sampling when
coupled with the METRIC stuff because hardware is daft.

And you can have SAMPLE_READ on non-leader events just fine.

> +
> +			/*
> +			 * The slots event must be before the metric events,
> +			 * because we only update the values of a topdown
> +			 * group once with the slots event.
> +			 */
> +			if (!is_slots_event(leader)) {
> +				for_each_sibling_event(sibling, leader) {
> +					if (is_slots_event(sibling))
> +						break;
> +					if (is_metric_event(sibling))
> +						return -EINVAL;
> +				}
> +			}

Per the SIBLING patch this then wants to be:

			if (!is_slots_event(leader))
				return -EINVAL;

			event->event_caps |= PERF_EV_CAP_SIBLING.
			/*
			 * Only once we have a METRICs sibling to we
			 * need TopDown magic.
			 */
			leader->hw.flags |= PERF_X86_EVENT_TOPDOWN;
> +		}
> +
> +		if (!is_sampling_event(event)) {
> +			if (event->attr.config1 != 0)
> +				return -EINVAL;

How does this depend on sampling?

> +			/*
> +			 * The TopDown metrics events and slots event don't
> +			 * support any filters.
> +			 */
> +			if (event->attr.config & X86_ALL_EVENT_FLAGS)
> +				return -EINVAL;

That seems independent of sampling too. Even a sampling SLOTS shouldn't
be having any of those afaict.

> +
> +			event->hw.flags |= PERF_X86_EVENT_TOPDOWN;

This is confusing too, a !sampling SLOTS event without METRIC siblings
shouldn't have this set, right? So arguably, this should be like above.

> +
> +			event->event_caps |= PERF_EV_CAP_COEXIST;
> +
> +			if (is_metric_event(event))
> +				event->hw.flags &= ~PERF_X86_EVENT_RDPMC_ALLOWED;

This too seems like something that should be in the is_metric_event()
branch above.

> +		}
> +	}
> +
>  	if (!(event->attr.config & ARCH_PERFMON_EVENTSEL_ANY))
>  		return 0;
>