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

From: Mark Rutland
Date: Mon Jan 11 2016 - 06:25:12 EST

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.