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SubjectRe: [PATCH v11 8/8] perf: ARM DynamIQ Shared Unit PMU support
On 03/02/2018 02:42 AM, Mark Rutland wrote:
> On Thu, Mar 01, 2018 at 12:35:49PM -0800, Saravana Kannan wrote:
>> On 03/01/2018 03:49 AM, Mark Rutland wrote:
>>> On Wed, Feb 28, 2018 at 02:17:33PM -0800, Saravana Kannan wrote:
>>>> 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.
>>>
>>> I believe that lock is currently superfluous, because the perf core
>>> ensures operations are cpu-affine, and have interrupts disabled in most
>>> cases (thanks to the context lock).
>>
>> I don't think it's superfluous. You have a common "event counter" selection
>> register and a common "event counter value" register. You can two CPUs
>> racing to read two unrelated event counters and end up causing one of them
>> to read a bogus value from the wrong event counter.
>
> It's important to note that the DSU PMU's event_init() ensures events
> are affine to a single CPU, and the perf core code serializes operations
> on those events via the context lock.

Ah, I see that now. Thanks!

> Therefore, two CPUs *won't* try to access the registers simultaneously.

Right, and this driver seems to be going through a lot of work to make
sure all events are read in one CPU.

Do you even have an upstream target where there are multiple DSU's in a
system? If not, we can simplify a ton of this code (no hotplug
notifiers, no migrating PMUs, no SMP calls, etc) by just adding a
spinlock and letting any CPU read these DSU counters.

If you need to support a system with multiple DSUs, I think it's still
useful to add CPU mask for events and letting the perf framework read
events on any of those CPUs.

> If events could be active on multiple CPUs simultaneously, I agree that
> the lock would be necessary. However, there would also be other problems
> to deal with in that case.
>
> If we want to allow pmu::read() from arbitrary CPUs the DSU is affine
> to, I agree we'd need the lock to serialize accesses to the registers
> and data structures.

Agreed.

So, depending on how many DSUs you want to support in the mainline
kernel, we can simplify it a ton. And if not, we can still try to remove
the need for smp calls so that we don't cause power impact when trying
to profile while measuring power.

Thanks,
Saravana

--
Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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