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

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Date:   Wed, 28 Feb 2018 14:17:33 -0800
From:   Saravana Kannan <skannan@...eaurora.org>
To:     Mark Rutland <mark.rutland@....com>
CC:     robh@...nel.org, mathieu.poirier@...aro.org,
        Suzuki K Poulose <suzuki.poulose@....com>,
        peterz@...radead.org, jonathan.cameron@...wei.com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        marc.zyngier@....com, sudeep.holla@....com, frowand.list@...il.com,
        leo.yan@...aro.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v11 8/8] perf: ARM DynamIQ Shared Unit PMU support

On 02/25/2018 06:36 AM, Mark Rutland wrote:
> 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.

You have to grab the dsu_pmu->pmu_lock spin lock anyway because the 
system registers are shared across all CPUs. So, just expanding it a bit 
to lock the hwc->prev_count and event->count updated doesn't seem to be 
any worse. In fact, it's better than sending pointless IPIs.

The local64_read/cmpxchg/add etc makes sense when you have per-cpu 
system registers like in the case of the ARM CPU PMU registers. It 
doesn't really buy us much for registers shared across the CPUs.

Thanks,
Saravana

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