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From: Patrick Bellasi <patrick.bellasi@arm.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Juri Lelli <juri.lelli@redhat.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	"open list:THERMAL" <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>,
	Paul Turner <pjt@google.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Todd Kjos <tkjos@google.com>, Joel Fernandes <joelaf@google.com>,
	"Cc: Steve Muckle" <smuckle@google.com>,
	surenb@google.com
Subject: Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks
Date: Mon, 13 Aug 2018 11:12:21 +0100
Message-ID: <20180813101221.GA2605@e110439-lin> (raw)
In-Reply-To: <CAKfTPtCWcq2bOmaC5EDdhS8ih8Sv7Mwzn=84xerKA2tY6kXLkw@mail.gmail.com>

Hi Vincent!

On 09-Aug 18:03, Vincent Guittot wrote:
> > On 07-Aug 15:26, Juri Lelli wrote:

[...]

> > > > +   util_cfs = cpu_util_cfs(rq);
> > > > +   util_rt  = cpu_util_rt(rq);
> > > > +   if (sched_feat(UCLAMP_SCHED_CLASS)) {
> > > > +           util = 0;
> > > > +           if (util_cfs)
> > > > +                   util += uclamp_util(cpu_of(rq), util_cfs);
> > > > +           if (util_rt)
> > > > +                   util += uclamp_util(cpu_of(rq), util_rt);
> > > > +   } else {
> > > > +           util  = cpu_util_cfs(rq);
> > > > +           util += cpu_util_rt(rq);
> > > > +           util  = uclamp_util(cpu_of(rq), util);
> > > > +   }
> >
> > Regarding the two policies, do you have any comment?
> 
> Does the policy for (sched_feat(UCLAMP_SCHED_CLASS)== true) really
> make sense as it is ?
> I mean, uclamp_util doesn't make any difference between rt and cfs
> tasks when clamping the utilization so why should be add twice the
> returned value ?
> IMHO, this policy would make sense if there were something like
> uclamp_util_rt() and a uclamp_util_cfs()

The idea for the UCLAMP_SCHED_CLASS policy is to improve fairness on
low-priority classese, especially when we have high RT utilization.

Let say we have:

 util_rt  = 40%, util_min=0%
 util_cfs = 10%, util_min=50%

the two policies will select:

  UCLAMP_SCHED_CLASS: util = uclamp(40) + uclamp(10) = 50 + 50   = 100%
 !UCLAMP_SCHED_CLASS: util = uclamp(40 + 10)         = uclmp(50) =  50%

Which means that, despite the CPU's util_min will be set to 50% when
CFS is running, these tasks will have almost no boost at all, since
their bandwidth margin is eclipsed by RT tasks.

> > We had an internal discussion and we found pro/cons for both... but

The UCLAMP_SCHED_CLASS policy is thus less energy efficiency but it
should grant a better "isolation" in terms of what is the expected
speed-up a task will get at run-time, independently from higher
priority classes.

Does that make sense?

> > I'm not sure keeping the sched_feat is a good solution on the long
> > run, i.e. mainline merge ;)

This problem still stands...

-- 
#include <best/regards.h>

Patrick Bellasi

  reply index

Thread overview: 82+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-06 16:39 [PATCH v3 00/14] Add utilization clamping support Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 01/14] sched/core: uclamp: extend sched_setattr to support utilization clamping Patrick Bellasi
2018-08-06 16:50   ` Randy Dunlap
2018-08-09  8:39     ` Patrick Bellasi
2018-08-09 15:20       ` Randy Dunlap
2018-08-07  9:59   ` Juri Lelli
2018-08-13 12:14     ` Patrick Bellasi
2018-08-13 12:27       ` Juri Lelli
2018-08-07 12:35   ` Juri Lelli
2018-08-09  9:14     ` Patrick Bellasi
2018-08-09  9:50       ` Juri Lelli
2018-08-09 15:23         ` Patrick Bellasi
2018-08-10  7:50           ` Juri Lelli
2018-08-17 10:34           ` Quentin Perret
2018-08-17 10:57             ` Patrick Bellasi
2018-08-17 11:14               ` Quentin Perret
2018-08-06 16:39 ` [PATCH v3 02/14] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups Patrick Bellasi
2018-08-14 11:25   ` Pavan Kondeti
2018-08-14 15:21     ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 03/14] sched/core: uclamp: add CPU's clamp groups accounting Patrick Bellasi
2018-08-14 15:44   ` Dietmar Eggemann
2018-08-14 16:49     ` Patrick Bellasi
2018-08-15  9:37       ` Dietmar Eggemann
2018-08-15 10:54         ` Patrick Bellasi
2018-08-15 10:59           ` Dietmar Eggemann
2018-08-16 13:32             ` Patrick Bellasi
2018-08-16 13:37               ` Quentin Perret
2018-08-16 13:45                 ` Dietmar Eggemann
2018-08-16 14:21                   ` Quentin Perret
2018-08-16 15:00                     ` Dietmar Eggemann
2018-08-17 11:04   ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 04/14] sched/core: uclamp: update CPU's refcount on clamp changes Patrick Bellasi
2018-08-15 15:02   ` Dietmar Eggemann
2018-08-16 13:22     ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 05/14] sched/cpufreq: uclamp: add utilization clamping for FAIR tasks Patrick Bellasi
2018-08-08 13:18   ` Vincent Guittot
2018-08-09 15:30     ` Patrick Bellasi
2018-08-15 15:30   ` Dietmar Eggemann
2018-08-16 13:53     ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks Patrick Bellasi
2018-08-07 13:26   ` Juri Lelli
2018-08-09 15:34     ` Patrick Bellasi
2018-08-09 16:03       ` Vincent Guittot
2018-08-13 10:12         ` Patrick Bellasi [this message]
2018-08-13 10:50           ` Juri Lelli
2018-08-13 12:07           ` Vincent Guittot
2018-08-13 12:09             ` Vincent Guittot
2018-08-13 12:49             ` Patrick Bellasi
2018-08-13 14:06               ` Vincent Guittot
2018-08-13 15:01                 ` Patrick Bellasi
2018-08-16 10:34                   ` Dietmar Eggemann
2018-08-16 13:40                     ` Patrick Bellasi
2018-08-07 13:54   ` Quentin Perret
2018-08-09 15:41     ` Patrick Bellasi
2018-08-09 15:55       ` Quentin Perret
2018-08-13 10:17         ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 07/14] sched/core: uclamp: enforce last task UCLAMP_MAX Patrick Bellasi
2018-08-16 15:43   ` Dietmar Eggemann
2018-08-16 16:47     ` Patrick Bellasi
2018-08-16 17:10       ` Dietmar Eggemann
2018-08-16 17:27         ` Patrick Bellasi
2018-08-16 17:20   ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 08/14] sched/core: uclamp: extend cpu's cgroup controller Patrick Bellasi
2018-08-17 12:21   ` Dietmar Eggemann
2018-08-17 14:24     ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 09/14] sched/core: uclamp: propagate parent clamps Patrick Bellasi
2018-08-16  9:09   ` Pavan Kondeti
2018-08-16 14:07     ` Patrick Bellasi
2018-08-17 13:43   ` Dietmar Eggemann
2018-08-17 14:45     ` Patrick Bellasi
2018-08-17 15:50       ` Dietmar Eggemann
2018-08-20 10:01         ` Dietmar Eggemann
2018-08-20 12:28           ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 10/14] sched/core: uclamp: map TG's clamp values into CPU's clamp groups Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 11/14] sched/core: uclamp: use TG's clamps to restrict Task's clamps Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 12/14] sched/core: uclamp: add system default clamps Patrick Bellasi
2018-08-16  9:13   ` Pavan Kondeti
2018-08-16 14:37     ` Patrick Bellasi
2018-08-20 10:18   ` Dietmar Eggemann
2018-08-20 12:27     ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 13/14] sched/core: uclamp: update CPU's refcount on TG's clamp changes Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 14/14] sched/core: uclamp: use percentage clamp values Patrick Bellasi

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