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

From: Suren Baghdasaryan <surenb@google.com>
To: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>, Tejun Heo <tj@kernel.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Paul Turner <pjt@google.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Juri Lelli <juri.lelli@redhat.com>, Todd Kjos <tkjos@google.com>,
	Joel Fernandes <joelaf@google.com>,
	Steve Muckle <smuckle@google.com>
Subject: Re: [PATCH v2 12/12] sched/core: uclamp: use percentage clamp values
Date: Tue, 24 Jul 2018 10:11:54 -0700	[thread overview]
Message-ID: <CAJuCfpHddNKXNVBs21sCADcHjW7zxxue8jg1B4KbPh-QLNe46g@mail.gmail.com> (raw)
In-Reply-To: <20180724164303.GB3162@e110439-lin>

On Tue, Jul 24, 2018 at 9:43 AM, Patrick Bellasi
<patrick.bellasi@arm.com> wrote:
> On 21-Jul 21:04, Suren Baghdasaryan wrote:
>> On Mon, Jul 16, 2018 at 1:29 AM, Patrick Bellasi
>> <patrick.bellasi@arm.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.
>

I think just (value & 0xFF) is enough to get you the right behavior.
~(value & 0x700) is not needed, it's effectively a NoOp which always
yields TRUE. For any *value* (value & 0x700) == 0x...00 and ~(value &
0x700) == 0x...FF == TRUE.

> 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