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Date:   Tue, 21 Jul 2020 14:40:42 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     kan.liang@linux.intel.com
Cc:     acme@redhat.com, mingo@kernel.org, linux-kernel@vger.kernel.org,
        jolsa@kernel.org, eranian@google.com,
        alexander.shishkin@linux.intel.com, ak@linux.intel.com
Subject: Re: [PATCH V6 09/14] perf/x86/intel: Support TopDown metrics on Ice
 Lake
Message-ID: <20200721124042.GX10769@hirez.programming.kicks-ass.net>
References: <20200717140554.22863-1-kan.liang@linux.intel.com>
 <20200717140554.22863-10-kan.liang@linux.intel.com>
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On Fri, Jul 17, 2020 at 07:05:49AM -0700, kan.liang@linux.intel.com wrote:

> +static inline u64 icl_get_metrics_event_value(u64 metric, u64 slots, int idx)
> +{
> +	u32 val;
> +
> +	/*
> +	 * The metric is reported as an 8bit integer fraction
> +	 * suming up to 0xff.
> +	 * slots-in-metric = (Metric / 0xff) * slots
> +	 */
> +	val = (metric >> ((idx - INTEL_PMC_IDX_METRIC_BASE) * 8)) & 0xff;
> +	return  mul_u64_u32_div(slots, val, 0xff);
> +}
> +
> +static void __icl_update_topdown_event(struct perf_event *event,
> +				       u64 slots, u64 metrics)
> +{
> +	int idx = event->hw.idx;
> +	u64 delta;
> +
> +	if (is_metric_idx(idx))
> +		delta = icl_get_metrics_event_value(metrics, slots, idx);
> +	else
> +		delta = slots;
> +
> +	local64_add(delta, &event->count);
> +}
> +
> +/*
> + * Update all active Topdown events.
> + *
> + * The PERF_METRICS and Fixed counter 3 are read separately. The values may be
> + * modify by a NMI. PMU has to be disabled before calling this function.
> + */
> +static u64 icl_update_topdown_event(struct perf_event *event)
> +{
> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> +	struct perf_event *other;
> +	u64 slots, metrics;
> +	int idx;
> +
> +	/* read Fixed counter 3 */
> +	rdpmcl((3 | INTEL_PMC_FIXED_RDPMC_BASE), slots);
> +	if (!slots)
> +		return 0;
> +
> +	/* read PERF_METRICS */
> +	rdpmcl(INTEL_PMC_FIXED_RDPMC_METRICS, metrics);
> +
> +	for_each_set_bit(idx, cpuc->active_mask, INTEL_PMC_IDX_TD_BE_BOUND + 1) {
> +		if (!is_topdown_idx(idx))
> +			continue;
> +		other = cpuc->events[idx];
> +		__icl_update_topdown_event(other, slots, metrics);
> +	}
> +
> +	/*
> +	 * Check and update this event, which may have been cleared
> +	 * in active_mask e.g. x86_pmu_stop()
> +	 */
> +	if (event && !test_bit(event->hw.idx, cpuc->active_mask))
> +		__icl_update_topdown_event(event, slots, metrics);
> +
> +	/*
> +	 * Software is recommended to periodically clear both registers
> +	 * in order to maintain accurate measurements, which is required for
> +	 * certain scenarios that involve sampling metrics at high rates.

I'll maintain that that statement is clear as mud and therefore useless.

> +	 * Software should always write fixed counter 3 before write to
> +	 * PERF_METRICS.
> +	 */
> +	wrmsrl(MSR_CORE_PERF_FIXED_CTR3, 0);
> +	wrmsrl(MSR_PERF_METRICS, 0);
> +
> +	return slots;
> +}

So in the normal case, this ends up calling into this function _5_
times, once for each event. Now the first time, it will actually do
something useful, but the other 4 times it's just wasting cycles.

Also, that for_each_set_bit() loop, trying to find the events to
update...

Can't we, instead, make the SLOTS update advance 5 running counters in
cpuc and feed the events off of that?