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Re: [PATCH v5 05/11] arm-cci PMU: Delay counter writes to pmu_enable

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On Mon, Jan 11, 2016 at 11:08:27AM +0000, Suzuki K. Poulose wrote:
> On 11/01/16 10:46, Mark Rutland wrote:
> >On Tue, Jan 05, 2016 at 09:59:13AM +0000, Suzuki K. Poulose wrote:
> >>On 04/01/16 19:24, Mark Rutland wrote:
> >>>On Mon, Jan 04, 2016 at 11:54:44AM +0000, Suzuki K. Poulose wrote:
> >>The pmu is not disabled while we are in overflow irq handler. Hence there may
> >>not be a pmu_enable() which would set the period for the counter which
> >>overflowed, if defer the write in that case. Is that assumption wrong ?
> >
> >As the driver stands today, yes.
> >
> >However, wouldn't it make more sense to disable the PMU for the overflow
> >handler, such that we can reuse the batching logic?
> 
> None of the PMU drivers do that AFAIK.

I see.

The Intel PMU driver disables the PMU for the interrupt handler; see
intel_pmu_handle_irq in arch/x86/kernel/cpu/perf_event_intel.c. It looks
like that's a special-case for sampling.

I guess we may have the only case where it makes sense to batch counter
writes as opposed to batching configuration writes.

> Hence, didn't want to change it for CCI. We could use the batching
> logic, if decide to do so. I can go ahead with that if there are no
> other side effects with that.

We'll lose events regardless as our RMW sequence will race against the
counters. Batching will make that window slightly larger, but other than
that I don't see a problem.

Thanks,
Mark.

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[PATCH v5 05/11] arm-cci PMU: Delay counter writes to pmu_enable "Suzuki K. Poulose" <suzuki.poulose@arm.com> - 2016-01-04 13:00 +0100
  Re: [PATCH v5 05/11] arm-cci PMU: Delay counter writes to pmu_enable Mark Rutland <mark.rutland@arm.com> - 2016-01-04 20:30 +0100
    Re: [PATCH v5 05/11] arm-cci PMU: Delay counter writes to pmu_enable "Suzuki K. Poulose" <Suzuki.Poulose@arm.com> - 2016-01-05 11:00 +0100
      Re: [PATCH v5 05/11] arm-cci PMU: Delay counter writes to pmu_enable Mark Rutland <mark.rutland@arm.com> - 2016-01-11 11:50 +0100
        Re: [PATCH v5 05/11] arm-cci PMU: Delay counter writes to pmu_enable "Suzuki K. Poulose" <Suzuki.Poulose@arm.com> - 2016-01-11 12:10 +0100
          Re: [PATCH v5 05/11] arm-cci PMU: Delay counter writes to pmu_enable Mark Rutland <mark.rutland@arm.com> - 2016-01-11 12:30 +0100
            Re: [PATCH v5 05/11] arm-cci PMU: Delay counter writes to pmu_enable "Suzuki K. Poulose" <Suzuki.Poulose@arm.com> - 2016-01-11 19:20 +0100

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