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From: Patrick Bellasi <patrick.bellasi@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	linux-api@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	Tejun Heo <tj@kernel.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Paul Turner <pjt@google.com>,
	Quentin Perret <quentin.perret@arm.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>,
	Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets refcounting
Date: Wed, 13 Mar 2019 11:37:57 +0000	[thread overview]
Message-ID: <20190313113757.aeaksz5akv6y5uep@e110439-lin> (raw)
In-Reply-To: <20190313081958.GA5996@hirez.programming.kicks-ass.net>

On 13-Mar 09:19, Peter Zijlstra wrote:
> 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.

Right, that don't match when UCLAMP_BUCKETS is a divider of
SCHED_CAPACITY_SCALE, i.e. when we use 8 or 16 buckets.

> > > 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.

Well, that 30% is really just ~16 utiliation units on a scale of 1024
when buckets have a size of 52.

Still, yes, we can argue that's big but that's also the same error
generated by DIV_ROUND_UP() when UCLAMP_BUCKETS is not 8 or 16.

> > 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().
                                                 ^^^^^^^^^^^^^^^^^^^^
This is different than DIV_ROUND_UP() and actually better across the
full range.

> 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.

I don't think we will have to cover other values in the further but I
agree that this "closest rounding" is definitively better.

Thanks for spotting it, will update in v8.

-- 
#include <best/regards.h>

Patrick Bellasi

  reply	other threads:[~2019-03-13 11:38 UTC|newest]

Thread overview: 84+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-08 10:05 [PATCH v7 00/15] Add utilization clamping support Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets refcounting Patrick Bellasi
2019-03-12 12:52   ` Dietmar Eggemann
2019-03-13 15:15     ` Patrick Bellasi
2019-03-13 21:01       ` Suren Baghdasaryan
2019-03-14 14:54         ` Patrick Bellasi
2019-03-14 15:00       ` Patrick Bellasi
2019-03-12 15:20   ` Peter Zijlstra
2019-03-12 15:50     ` Patrick Bellasi
2019-03-13  8:19       ` Peter Zijlstra
2019-03-13 11:37         ` Patrick Bellasi [this message]
2019-03-13 13:40   ` Peter Zijlstra
2019-03-13 16:12     ` Patrick Bellasi
2019-03-13 17:22       ` Peter Zijlstra
2019-03-13 18:22         ` Patrick Bellasi
2019-03-13 19:48       ` Peter Zijlstra
2019-03-14 12:13         ` Patrick Bellasi
2019-03-14 13:32           ` Peter Zijlstra
2019-03-14 15:07             ` Patrick Bellasi
2019-03-14 19:18               ` Peter Zijlstra
2019-03-13 13:52   ` Peter Zijlstra
2019-03-13 15:59     ` Patrick Bellasi
2019-03-13 19:30       ` Peter Zijlstra
2019-03-14 11:03         ` Patrick Bellasi
2019-03-14 13:27           ` Peter Zijlstra
2019-03-13 19:39       ` Peter Zijlstra
2019-03-14 11:18         ` Patrick Bellasi
2019-03-13 21:23     ` Suren Baghdasaryan
2019-03-14 12:43       ` Patrick Bellasi
2019-03-13 14:06   ` Peter Zijlstra
2019-03-13 15:28     ` Patrick Bellasi
2019-03-13 14:09   ` Peter Zijlstra
2019-03-13 15:23     ` Patrick Bellasi
2019-03-13 19:46       ` Peter Zijlstra
2019-03-13 21:08         ` Suren Baghdasaryan
2019-03-14 12:22           ` Patrick Bellasi
2019-03-14 11:45         ` Patrick Bellasi
2019-03-13 21:32   ` Suren Baghdasaryan
2019-03-14 14:46     ` Patrick Bellasi
2019-03-14 15:29       ` Suren Baghdasaryan
2019-03-14 15:40         ` Patrick Bellasi
2019-03-14 16:39           ` Suren Baghdasaryan
2019-02-08 10:05 ` [PATCH v7 02/15] sched/core: uclamp: Enforce last task UCLAMP_MAX Patrick Bellasi
2019-03-13 14:10   ` Peter Zijlstra
2019-03-13 16:20     ` Patrick Bellasi
2019-03-13 17:29       ` Peter Zijlstra
2019-03-13 18:29         ` Patrick Bellasi
2019-03-13 14:12   ` Peter Zijlstra
2019-03-13 16:16     ` Patrick Bellasi
2019-03-14  0:29       ` Suren Baghdasaryan
2019-03-14 17:06         ` Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 03/15] sched/core: uclamp: Add system default clamps Patrick Bellasi
2019-03-13 14:32   ` Peter Zijlstra
2019-03-13 17:09     ` Patrick Bellasi
2019-03-13 19:58       ` Peter Zijlstra
2019-03-13 20:10       ` Peter Zijlstra
2019-03-15 13:41         ` Patrick Bellasi
2019-03-13 20:13   ` Peter Zijlstra
2019-03-13 20:18   ` Peter Zijlstra
2019-03-18 12:18     ` Patrick Bellasi
2019-03-18 13:10       ` Peter Zijlstra
2019-03-18 14:21         ` Patrick Bellasi
2019-03-18 14:29           ` Peter Zijlstra
2019-02-08 10:05 ` [PATCH v7 04/15] sched/core: Allow sched_setattr() to use the current policy Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 05/15] sched/core: uclamp: Extend sched_setattr() to support utilization clamping Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 06/15] sched/core: uclamp: Reset uclamp values on RESET_ON_FORK Patrick Bellasi
2019-03-13 20:52   ` Peter Zijlstra
2019-03-18 12:58     ` Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 07/15] sched/core: uclamp: Set default clamps for RT tasks Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 08/15] sched/cpufreq: uclamp: Add clamps for FAIR and " Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 09/15] sched/core: uclamp: Add uclamp_util_with() Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 10/15] sched/fair: uclamp: Add uclamp support to energy_compute() Patrick Bellasi
2019-03-06 17:21   ` Quentin Perret
2019-03-18 15:19     ` Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 11/15] sched/core: uclamp: Extend CPU's cgroup controller Patrick Bellasi
2019-02-14 15:48   ` Tejun Heo
2019-03-19 10:00     ` Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 12/15] sched/core: uclamp: Propagate parent clamps Patrick Bellasi
2019-03-14 16:17   ` Suren Baghdasaryan
2019-03-18 16:54     ` Patrick Bellasi
2019-03-18 16:58       ` Suren Baghdasaryan
2019-02-08 10:05 ` [PATCH v7 13/15] sched/core: uclamp: Propagate system defaults to root group Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 14/15] sched/core: uclamp: Use TG's clamps to restrict TASK's clamps Patrick Bellasi
2019-02-08 10:05 ` [PATCH v7 15/15] sched/core: uclamp: Update CPU's refcount on TG's clamp changes Patrick Bellasi

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