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

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Date:   Fri, 19 Jun 2020 12:57:23 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Valentin Schneider <valentin.schneider@....com>
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 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.

-- 
Mel Gorman
SUSE Labs