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Date:   Fri, 19 Jun 2020 15:51:23 +0100
From:   Qais Yousef <qais.yousef@arm.com>
To:     Mel Gorman <mgorman@suse.de>
Cc:     Valentin Schneider <valentin.schneider@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Patrick Bellasi <patrick.bellasi@matbug.net>,
        Chris Redpath <chrid.redpath@arm.com>,
        Lukasz Luba <lukasz.luba@arm.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] sched/uclamp: Protect uclamp fast path code with
 static key
Message-ID: <20200619145113.tgaxvzejmpstsnx7@e107158-lin.cambridge.arm.com>
References: <20200618195525.7889-1-qais.yousef@arm.com>
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 <jhjwo43cpfl.mognet@arm.com>
 <20200619115723.GF3129@suse.de>
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On 06/19/20 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. 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.

The diff between nouclamp and uclamp-static-key (disabling uclamp in fast path)

     8.73%     -1.55%  [kernel.kallsyms]        [k] try_to_wake_up
     0.07%     +0.04%  [kernel.kallsyms]        [k] deactivate_task
     0.13%     -0.02%  [kernel.kallsyms]        [k] activate_task

The diff between nouclamp and uclamp-static-key (uclamp actively used in the
fast path)

     8.73%     -0.72%  [kernel.kallsyms]        [k] try_to_wake_up
     0.13%     +0.39%  [kernel.kallsyms]        [k] activate_task
     0.07%     +0.38%  [kernel.kallsyms]        [k] deactivate_task

I will include these numbers in the commit message in v2.

Thanks

--
Qais Yousef