linux-kernel - Re: [PATCH v2 12/12] sched/core: uclamp: use percentage clamp values

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Date:   Tue, 24 Jul 2018 17:43:03 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.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>
Subject: Re: [PATCH v2 12/12] sched/core: uclamp: use percentage clamp values

On 21-Jul 21:04, Suren Baghdasaryan wrote:
> On Mon, Jul 16, 2018 at 1:29 AM, Patrick Bellasi
> <patrick.bellasi@....com> wrote:

[...]

> > +static inline unsigned int scale_from_percent(unsigned int pct)
> > +{
> > +       WARN_ON(pct > 100);
> > +
> > +       return ((SCHED_FIXEDPOINT_SCALE * pct) / 100);
> > +}
> > +
> > +static inline unsigned int scale_to_percent(unsigned int value)
> > +{
> > +       unsigned int rounding = 0;
> > +
> > +       WARN_ON(value > SCHED_FIXEDPOINT_SCALE);
> > +
> > +       /* Compensate rounding errors for: 0, 256, 512, 768, 1024 */
> > +       if (likely((value & 0xFF) && ~(value & 0x700)))
> > +               rounding = 1;
> 
> Hmm. I don't think ~(value & 0x700) will ever yield FALSE... What am I missing?

So, 0x700 is the topmost 3 bits sets (111 0000 0000) which different
configuration corresponds to:

 001 0000 0000 =>  256
 010 0000 0000 =>  512
 011 0000 0000 =>  768
 100 0000 0000 => 1024

Thus, if 0x700 matches then we have one of these values in input and
for these cases we have to add a unit to the percentage value.

For the case (value == 0) we translate it into 0% thanks to the check
on (value & 0xFF) to ensure rounding = 0.

Here is a small python snippet I've used to check the conversion of
all the possible percentage values:

---8<---
values = range(0, 101)
for pct in xrange(0, 101):
	util = int((1024 * pct) / 100)
        rounding = 1
        if not ((util & 0xFF) and ~(util & 0x700)):
            print "Fixing util_to_perc({:3d} => {:4d})".format(pct, util)
            rounding = 0
	pct2 = (rounding + ((100 * util) / 1024))
	if pct2 in values:
		values.remove(pct2)
        if pct != pct2:
            print "Convertion failed for: {:3d} => {:4d} => {:3d}".format(pct, util, pct2)
if values:
    print "ERROR: not all percentage values converted"
---8<---

-- 
#include <best/regards.h>

Patrick Bellasi