linux-kernel - Re: [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets refcounting

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Date:   Wed, 13 Mar 2019 09:19:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets
 refcounting

On Tue, Mar 12, 2019 at 03:50:43PM +0000, Patrick Bellasi wrote:
> On 12-Mar 16:20, Peter Zijlstra wrote:
> > On Fri, Feb 08, 2019 at 10:05:40AM +0000, Patrick Bellasi wrote:
> > > +/* Integer ceil-rounded range for each bucket */

          ^^^^^^^^^^^^^^^^^^^^^^^^^^^

> > > +#define UCLAMP_BUCKET_DELTA ((SCHED_CAPACITY_SCALE / UCLAMP_BUCKETS) + 1)

                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

simply do not match.

> > Uhm, should that not me ((x+y-1)/y), aka. DIV_ROUND_UP(x,y) ?
> 
> Well, there is certainly some rounding to be done...
> 
> > The above would give 4 for 9/3, which is clearly buggered.
> 
> .. still the math above should work fine within the boundaries we
> define for UCLAMP_BUCKET_DELTA (5..20 groups) and considering that
> SCHED_CAPACITY_SCALE will never be smaller then 1024.

That's a very poor reason to write utter nonsense :-)

> The above is designed to shrink the topmost bucket wrt all the others
> but it will never be smaller than ~30%.

30% sounds like a lot, esp. for this range.

> Here are the start values computed for each bucket using the math
> above and the computed shrinking percentage for the topmost bucket:

If you use a regular rounding, the error is _much_ smaller:

$ for ((x=5;x<21;x++)) ; do let d=(1024+x/2)/x; let s=(x-1)*d; let e=1024-s; let p=100*(d-e)/d; echo $x $d $s $e $p%; done
5 205 820 204 0%
6 171 855 169 1%
7 146 876 148 -1%
8 128 896 128 0%
9 114 912 112 1%
10 102 918 106 -3%
11 93 930 94 -1%
12 85 935 89 -4%
13 79 948 76 3%
14 73 949 75 -2%
15 68 952 72 -5%
16 64 960 64 0%
17 60 960 64 -6%
18 57 969 55 3%
19 54 972 52 3%
20 51 969 55 -7%

Funnily enough, we have a helper for that too: DIV_ROUND_CLOSEST().

Now, if we go further, the error will obviously increase because we run
out of precision, but even there, regular rounding will be better than
either floor or ceil.