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From: Dietmar Eggemann <dietmar.eggemann@arm.com>
To: Patrick Bellasi <patrick.bellasi@arm.com>,
	Vincent Guittot <vincent.guittot@linaro.org>
Cc: Juri Lelli <juri.lelli@redhat.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-pm <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>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Todd Kjos <tkjos@google.com>, Joel Fernandes <joelaf@google.com>,
	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: Thu, 16 Aug 2018 12:34:05 +0200
Message-ID: <1b72b94c-5411-4b95-01a6-49ac978acbd5@arm.com> (raw)
In-Reply-To: <20180813150112.GE2605@e110439-lin>

On 08/13/2018 05:01 PM, Patrick Bellasi wrote:
> On 13-Aug 16:06, Vincent Guittot wrote:
>> On Mon, 13 Aug 2018 at 14:49, Patrick Bellasi <patrick.bellasi@arm.com> wrote:
>>> On 13-Aug 14:07, Vincent Guittot wrote:
>>>> On Mon, 13 Aug 2018 at 12:12, Patrick Bellasi <patrick.bellasi@arm.com> wrote:
> 
> [...]
> 
>>> 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?
>>
>> As said above, I don't think that there is a problem that is specific
>> to cross class scheduling that can't also happen in the same class.
>>
>> Regarding your example:
>> task TA util=40% with uclamp_min 50%
>> task TB util=10% with uclamp_min 0%
>>
>> If TA and TB are cfs, util=50% and it doesn't seem to be a problem
>> whereas TB will steal some bandwidth to TA and delay it (and i even
>> don't speak about the impact of the nice priority of TB)
>> If TA is cfs and TB is rt, Why util=50% is now a problem for TA ?
> 
> You right, in the current implementation, where we _do not_
> distinguish among scheduling classes it's not possible to get a
> reasonable implementation of a per sched class clamping.
> 
>>> To a certain extend I see this problem similar to the rt/dl/irq pressure
>>> in defining cpu_capacity, isn't it?
> 
> However, I still think that higher priority classes eclipsing the
> clamping of lower priority classes can still be a problem.
> 
> In your example above, the main difference between TA and TB being on
> the same class or different classes is that in the second case TB
> is granted to always preempt TA. We can end up with a non boosted RT
> task consuming all the boosted bandwidth required by a CFS task.
> 
> This does not happen, apart maybe for the corner case of really
> different nice values, if the tasks are both CFS, since the fair
> scheduler will grant some progress for both of them.
> 
> Thus, given the current implementation, I think it makes sense to drop
> the UCLAMP_SCHED_CLASS policy and stick with a more clean and
> consistent design.

I agree with everything said in this thread so far.
So in case you skip UCLAMP_SCHED_CLASS [(B) combine the clamped 
utilizations] in v4, you will only provide [A) clamp the combined 
utilization]?

I assume that we don't have to guard the util clamping for rt tasks 
behind a disabled by default sched feature because all runnable rt tasks 
will have util_min = SCHED_CAPACITY_SCALE by default?

> I'll then see if it makes sense to add a dedicated patch on top of the
> series to add a proper per-class clamp tracking.

I assume if you introduce this per-class clamping you will switch to use 
the UCLAMP_SCHED_CLASS approach?

  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
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 [this message]
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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