linux-kernel - Re: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics

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Date:   Tue, 28 May 2019 15:30:22 +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: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics

On Tue, May 21, 2019 at 02:40:50PM -0700, kan.liang@...ux.intel.com wrote:
> +static u64 icl_metric_update_event(struct perf_event *event, u64 val)
> +{
> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> +	struct hw_perf_event *hwc = &event->hw;
> +	u64 newval, metric, slots_val = 0, new, last;
> +	bool nmi = in_nmi();
> +	int txn_flags = nmi ? 0 : cpuc->txn_flags;
> +
> +	/*
> +	 * Use cached value for transaction.
> +	 */
> +	newval = 0;
> +	if (txn_flags) {
> +		newval = cpuc->txn_metric;
> +		slots_val = cpuc->txn_slots;
> +	} else if (nmi) {
> +		newval = cpuc->nmi_metric;
> +		slots_val = cpuc->nmi_slots;
> +	}
> +
> +	if (!newval) {
> +		slots_val = val;
> +
> +		rdpmcl((1<<29), newval);
> +		if (txn_flags) {
> +			cpuc->txn_metric = newval;
> +			cpuc->txn_slots = slots_val;
> +		} else if (nmi) {
> +			cpuc->nmi_metric = newval;
> +			cpuc->nmi_slots = slots_val;
> +		}
> +
> +		if (!(txn_flags & PERF_PMU_TXN_REMOVE)) {
> +			/* Reset the metric value when reading
> +			 * The SLOTS register must be reset when PERF_METRICS reset,
> +			 * otherwise PERF_METRICS may has wrong output.
> +			 */

broken comment style.. (and grammer)

> +			wrmsrl(MSR_PERF_METRICS, 0);
> +			wrmsrl(MSR_CORE_PERF_FIXED_CTR3, 0);

I don't get this, overflow happens on when we flip sign, so why is
programming 0 a sane thing to do?

> +			hwc->saved_metric = 0;
> +			hwc->saved_slots = 0;
> +		} else {
> +			/* saved metric and slots for context switch */
> +			hwc->saved_metric = newval;
> +			hwc->saved_slots = val;
> +
> +		}
> +		/* cache the last metric and slots */
> +		cpuc->last_metric = hwc->last_metric;
> +		cpuc->last_slots = hwc->last_slots;
> +		hwc->last_metric = 0;
> +		hwc->last_slots = 0;
> +	}
> +
> +	/* The METRICS and SLOTS have been reset when reading */
> +	if (!(txn_flags & PERF_PMU_TXN_REMOVE))
> +		local64_set(&hwc->prev_count, 0);
> +
> +	if (is_slots_event(event))
> +		return (slots_val - cpuc->last_slots);
> +
> +	/*
> +	 * The metric is reported as an 8bit integer percentage
> +	 * suming up to 0xff. As the counter is less than 64bits
> +	 * we can use the not used bits to get the needed precision.
> +	 * Use 16bit fixed point arithmetic for
> +	 * slots-in-metric = (MetricPct / 0xff) * val
> +	 * This works fine for upto 48bit counters, but will
> +	 * lose precision above that.
> +	 */
> +
> +	metric = (cpuc->last_metric >> ((hwc->idx - INTEL_PMC_IDX_FIXED_METRIC_BASE)*8)) & 0xff;
> +	last = (((metric * 0xffff) >> 8) * cpuc->last_slots) >> 16;

How is that cpuc->last_* crap not broken for NMIs ?

> +
> +	metric = (newval >> ((hwc->idx - INTEL_PMC_IDX_FIXED_METRIC_BASE)*8)) & 0xff;
> +	new = (((metric * 0xffff) >> 8) * slots_val) >> 16;
> +
> +	return (new - last);
> +}


This is diguisting.. and unreadable.

mul_u64_u32_shr() is actually really fast, use it.