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

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Message-ID: <20160105145319.GO6344@twins.programming.kicks-ass.net>
Date:	Tue, 5 Jan 2016 15:53:19 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	mark.rutland@....com, punit.agrawal@....com, arm@...nel.org
Subject: Re: [PATCH v4 05/12] arm-cci: PMU: Add support for transactions

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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