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>,
Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks
Date: Mon, 13 Aug 2018 13:49:11 +0100 [thread overview]
Message-ID: <20180813124911.GD2605@e110439-lin> (raw)
In-Reply-To: <CAKfTPtBN+K=8fqukZix2k0rdcjG8YiYagxZtzLx7kk3US+9V2Q@mail.gmail.com>
On 13-Aug 14:07, Vincent Guittot wrote:
> On Mon, 13 Aug 2018 at 12:12, Patrick Bellasi <patrick.bellasi@arm.com> wrote:
> >
> > 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.
>
> Hmm ... At the opposite, even if there is no running rt task but only
> some remaining blocked rt utilization,
> even if util_rt = 10%, util_min=0%
> and util_cfs = 40%, util_min=50%
> the UCLAMP_SCHED_CLASS: util = uclamp(10) + uclamp(40) = 50 + 50 = 100%
Yes, that's true... since now I clamp util_rt if it's non zero.
Perhaps this can be fixed by clamping util_rt only:
if (rt_rq_is_runnable(&rq->rt))
?
> So cfs task can get double boosted by a small rt task.
Well, in principle we don't know if the 50% clamp was asserted by the
RT or the CFS task, since in the current implementation we max
aggregate clamp values across all RT and CFS tasks.
> Furthermore, if there is no rt task but 2 cfs tasks of 40% and 10%
> the UCLAMP_SCHED_CLASS: util = uclamp(0) + uclamp(40) = 50 = 50%
True, but here we are within the same class and what utilization
clamping aims to do is to defined the minimum capacity to run _all_
the RUNNABLE tasks... not the minimum capacity for _each_ one of them.
> So in this case cfs tasks don't get more boost and have to share the
> bandwidth and you don't ensure 50% for each unlike what you try to do
> for rt.
Above I'm not trying to fix a per-task issue. The UCLAMP_SCHED_CLASS
policy is just "trying" to fix a cross-class issue... if we agree
there can be a cross-class issue worth to be fixed.
> You create a difference in the behavior depending of the class of the
> others co-schedule tasks which is not sane IMHO
Yes I agree that the current behavior is not completely clean... still
the question is: do you reckon the problem I depicted above, i.e. RT
workloads eclipsing the min_util required by lower priority classes?
To a certain extend I see this problem similar to the rt/dl/irq pressure
in defining cpu_capacity, isn't it?
Maybe we can make use of (cpu_capacity_orig - cpu_capacity) to factor
in a util_min compensation for CFS tasks?
--
#include <best/regards.h>
Patrick Bellasi
next prev parent reply other threads:[~2018-08-13 12:49 UTC|newest]
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
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 [this message]
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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