Messages in this thread | | | Date | Fri, 02 Mar 2018 11:19:56 -0800 | From | Saravana Kannan <> | Subject | Re: [PATCH v11 8/8] perf: ARM DynamIQ Shared Unit PMU support |
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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
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