Re: [PATCH v4 05/12] arm-cci: PMU: Add support for transactions

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH v4 05/12] arm-cci: PMU: Add support for transactions
Date	2016-01-05 16:00 +0100
Message-ID	<qNBfA-4bp-5@gated-at.bofh.it> (permalink)
References	(5 earlier) <qH0Vs-4xe-9@gated-at.bofh.it> <qH1HP-55K-13@gated-at.bofh.it> <qNA0a-3p1-3@gated-at.bofh.it> <qNA0a-3p1-1@gated-at.bofh.it> <qNA9R-3si-19@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Tue, Jan 05, 2016 at 01:43:30PM +0000, Suzuki K. Poulose wrote:
> On 05/01/16 13:37, Peter Zijlstra wrote:
> >On Mon, Dec 21, 2015 at 10:55:29AM +0000, Suzuki K. Poulose wrote:
> >>Thanks for that hint. Here is what I cam up with. We don't reschedule
> >>the events, all we need to do is group the writes to the counters. Hence
> >>we could as well add a flag for those events which need programming
> >>and perform the write in pmu::pmu_enable().
> >
> >I'm still somewhat confused..
> >
> >>Grouping the writes to counters can ammortise the cost of the operation
> >>on PMUs where it is expensive (e.g, CCI-500).
> >
> >This rationale makes me think you want to reduce the number of counter
> >writes, not batch them per-se.
> >
> >So why are you unconditionally writing all counters, instead of only
> >those that changed?
> >
> 
> The ARM CCI PMU reprograms all the counters with a specific value (2^31)
> to account for high interrupt latencies in recording the counters that
> overflowed. So, pmu_stop() updates the counter and pmu_start() resets
> the counter to the above value, always.
> 
> Now, writing to a single counter requires
> 
> 1) Stopping and disabling all the counters in HW (So that step 3 doesn't
> interfere with the other counters)
> 2) Program the target counter with invalid event and enable the counter.
> 3) Enable the PMU and then write to the counter.
> 4) Reset everything back to normal.
> 
> 
> So, the approach here is to delay the writes to the counters as much as possible
> and batch them. So that we don't have to repeat steps 1 & 4 for every single
> counter.
> 
> Does it help ?

Yes, thanks!
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Re: [PATCH v4 05/12] arm-cci: PMU: Add support for transactions Peter Zijlstra <peterz@infradead.org> - 2016-01-05 14:40 +0100
  Re: [PATCH v4 05/12] arm-cci: PMU: Add support for transactions "Suzuki K. Poulose" <Suzuki.Poulose@arm.com> - 2016-01-05 14:50 +0100
    Re: [PATCH v4 05/12] arm-cci: PMU: Add support for transactions Peter Zijlstra <peterz@infradead.org> - 2016-01-05 16:00 +0100

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