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SubjectRe: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics
On Tue, May 21, 2019 at 02:40:50PM -0700, kan.liang@linux.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.

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