Re: [PATCH v4] perf/x86/amd/power: Add AMD accumulated power reporting mechanism

[Peter Zijlstra]

On Thu, Jan 28, 2016 at 10:03:15AM +0100, Borislav Petkov wrote:

> +
> +struct power_pmu {
> +	raw_spinlock_t		lock;

Now that the list is gone, what does this thing protect?

> +	struct pmu		*pmu;

This member seems superfluous, there's only the one possible value.

> +	local64_t		cpu_sw_pwr_ptsc;
> +
> +	/*
> +	 * These two cpumasks are used for avoiding the allocations on the
> +	 * CPU_STARTING phase because power_cpu_prepare() will be called with
> +	 * IRQs disabled.
> +	 */
> +	cpumask_var_t		mask;
> +	cpumask_var_t		tmp_mask;
> +};
> +
> +static struct pmu pmu_class;
> +
> +/*
> + * Accumulated power represents the sum of each compute unit's (CU) power
> + * consumption. On any core of each CU we read the total accumulated power from
> + * MSR_F15H_CU_PWR_ACCUMULATOR. cpu_mask represents CPU bit map of all cores
> + * which are picked to measure the power for the CUs they belong to.
> + */
> +static cpumask_t cpu_mask;
> +
> +static DEFINE_PER_CPU(struct power_pmu *, amd_power_pmu);
> +
> +static u64 event_update(struct perf_event *event, struct power_pmu *pmu)
> +{

Is there ever a case where @pmu != __this_cpu_read(power_pmu) ?

> +	struct hw_perf_event *hwc = &event->hw;
> +	u64 prev_raw_count, new_raw_count, prev_ptsc, new_ptsc;
> +	u64 delta, tdelta;
> +
> +again:
> +	prev_raw_count = local64_read(&hwc->prev_count);
> +	prev_ptsc = local64_read(&pmu->cpu_sw_pwr_ptsc);
> +	rdmsrl(event->hw.event_base, new_raw_count);

Is hw.event_base != MSR_F15H_CU_PWR_ACCUMULATOR possible?

> +	rdmsrl(MSR_F15H_PTSC, new_ptsc);


Also, I suspect this doesn't do what you expect it to do.

We measure per-event PWR_ACC deltas, but per CPU PTSC values. These do
not match when there's more than 1 event on the CPU.

I would suggest adding a new struct to the hw_perf_event union with the
two u64 deltas like:

	struct { /* amd_power */
		u64 pwr_acc;
		u64 ptsc;
	};

And track these values per-event.

> +
> +	if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
> +			    new_raw_count) != prev_raw_count) {
> +		cpu_relax();
> +		goto again;
> +	}
> +
> +	/*
> +	 * Calculate the CU power consumption over a time period, the unit of
> +	 * final value (delta) is micro-Watts. Then add it to the event count.
> +	 */
> +	if (new_raw_count < prev_raw_count) {
> +		delta = max_cu_acc_power + new_raw_count;
> +		delta -= prev_raw_count;
> +	} else
> +		delta = new_raw_count - prev_raw_count;
> +
> +	delta *= cpu_pwr_sample_ratio * 1000;
> +	tdelta = new_ptsc - prev_ptsc;
> +
> +	do_div(delta, tdelta);
> +	local64_add(delta, &event->count);

Then this division can be redone on the total values, that looses less
precision over-all.

> +
> +	return new_raw_count;
> +}