linux-kernel - Re: [PATCH 2/2] sched/uclamp: Protect uclamp fast path code with static key

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Message-ID: <jhjtuz7ckrr.mognet@arm.com>
Date:   Fri, 19 Jun 2020 13:17:28 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Qais Yousef <qais.yousef@....com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Chris Redpath <chrid.redpath@....com>,
        Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] sched/uclamp: Protect uclamp fast path code with static key


On 19/06/20 12:57, Mel Gorman wrote:
> On Fri, Jun 19, 2020 at 11:36:46AM +0100, Valentin Schneider wrote:
>> >                                    nouclamp                 uclamp      uclamp-static-key
>> > Hmean     send-64         162.43 (   0.00%)      157.84 *  -2.82%*      163.39 *   0.59%*
>> > Hmean     send-128        324.71 (   0.00%)      314.78 *  -3.06%*      326.18 *   0.45%*
>> > Hmean     send-256        641.55 (   0.00%)      628.67 *  -2.01%*      648.12 *   1.02%*
>> > Hmean     send-1024      2525.28 (   0.00%)     2448.26 *  -3.05%*     2543.73 *   0.73%*
>> > Hmean     send-2048      4836.14 (   0.00%)     4712.08 *  -2.57%*     4867.69 *   0.65%*
>> > Hmean     send-3312      7540.83 (   0.00%)     7425.45 *  -1.53%*     7621.06 *   1.06%*
>> > Hmean     send-4096      9124.53 (   0.00%)     8948.82 *  -1.93%*     9276.25 *   1.66%*
>> > Hmean     send-8192     15589.67 (   0.00%)    15486.35 *  -0.66%*    15819.98 *   1.48%*
>> > Hmean     send-16384    26386.47 (   0.00%)    25752.25 *  -2.40%*    26773.74 *   1.47%*
>> >
>>
>> Am I reading this correctly in that compiling in uclamp but having the
>> static key enabled gives a slight improvement compared to not compiling in
>> uclamp? I suppose the important bit is that we're not seeing regressions
>> anymore, but still.
>>
>
> I haven't reviewed the series in depth because from your review, another
> version is likely in the works.

I don't wait Qais to hate me here - I think you could start the performance
testing on this version if you feel like it, given my comments were mostly
on changelog / debug options - the core of that patch shouldn't change
much.

> However, it is not that unusual to
> see small fluctuations like this that are counter-intuitive. The report
> indicates the difference is likely outside of the noise with * around the
> percentage difference instead of () but it could be small boot-to-boot
> variance, differences in code layout, slight differences in slab usage
> patterns etc. The definitive evidence that uclamp overhead is no there
> is whether the uclamp functions show up in annotated profiles or not.

I see, thanks! I suppose if we have access to individual samples we can
also run some statistical tests / stare at some boxplots to see how it
compares.