linux-kernel - Re: [PATCH v11 8/8] perf: ARM DynamIQ Shared Unit PMU support

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Date:   Sun, 25 Feb 2018 14:36:53 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Saravana Kannan <skannan@...eaurora.org>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>, will.deacon@....com,
        robh@...nel.org, sudeep.holla@....com, mathieu.poirier@...aro.org,
        peterz@...radead.org, jonathan.cameron@...wei.com,
        linux-kernel@...r.kernel.org, marc.zyngier@....com,
        leo.yan@...aro.org, frowand.list@...il.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v11 8/8] perf: ARM DynamIQ Shared Unit PMU support

On Fri, Feb 23, 2018 at 04:53:18PM -0800, Saravana Kannan wrote:
> On 01/02/2018 03:25 AM, Suzuki K Poulose wrote:
> > +static void dsu_pmu_event_update(struct perf_event *event)
> > +{
> > +	struct hw_perf_event *hwc = &event->hw;
> > +	u64 delta, prev_count, new_count;
> > +
> > +	do {
> > +		/* We may also be called from the irq handler */
> > +		prev_count = local64_read(&hwc->prev_count);
> > +		new_count = dsu_pmu_read_counter(event);
> > +	} while (local64_cmpxchg(&hwc->prev_count, prev_count, new_count) !=
> > +			prev_count);
> > +	delta = (new_count - prev_count) & DSU_PMU_COUNTER_MASK(hwc->idx);
> > +	local64_add(delta, &event->count);
> > +}
> > +
> > +static void dsu_pmu_read(struct perf_event *event)
> > +{
> > +	dsu_pmu_event_update(event);
> > +}

> I sent out a patch that'll allow PMUs to set an event flag to avoid
> unnecessary smp calls when the event can be read from any CPU. You could
> just always set that if you can't have multiple DSU's running the kernel (I
> don't know if the current ARM designs support having multiple DSUs in a
> SoC/system) or set it if associated_cpus == cpu_present_mask.

As-is, that won't be safe, given the read function calls the event_update()
function, which has side-effects on hwc->prec_count and event->count. Those
need to be serialized somehow.

Thanks,
Mark.