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From: Suren Baghdasaryan <surenb@google.com>
To: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: LKML <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>,
	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>
Subject: Re: [PATCH v4 09/16] sched/core: uclamp: map TG's clamp values into CPU's clamp groups
Date: Wed, 12 Sep 2018 08:53:39 -0700
Message-ID: <CAJuCfpH9VCZjmXG4vDt7JtExwhnLdNwUJhUpfaz43owm2UrAPg@mail.gmail.com> (raw)
In-Reply-To: <20180912141923.GF1413@e110439-lin>

On Wed, Sep 12, 2018 at 7:19 AM, Patrick Bellasi
<patrick.bellasi@arm.com> wrote:
> On 09-Sep 11:52, Suren Baghdasaryan wrote:
>> On Tue, Aug 28, 2018 at 6:53 AM, Patrick Bellasi
>> <patrick.bellasi@arm.com> wrote:
>
> [...]
>
>> > +/**
>> > + * release_uclamp_sched_group: release utilization clamp references of a TG
>>
>> free_uclamp_sched_group
>
> +1
>
>> > + * @tg: the task group being removed
>> > + *
>> > + * An empty task group can be removed only when it has no more tasks or child
>> > + * groups. This means that we can also safely release all the reference
>> > + * counting to clamp groups.
>> > + */
>> > +static inline void free_uclamp_sched_group(struct task_group *tg)
>> > +{
>
> [...]
>
>> > @@ -1417,9 +1444,18 @@ static void __init init_uclamp(void)
>> >  #ifdef CONFIG_UCLAMP_TASK_GROUP
>> >                 /* Init root TG's clamp group */
>> >                 uc_se = &root_task_group.uclamp[clamp_id];
>> > +
>> >                 uc_se->effective.value = uclamp_none(clamp_id);
>> > -               uc_se->value = uclamp_none(clamp_id);
>> > -               uc_se->group_id = 0;
>> > +               uc_se->effective.group_id = 0;
>> > +
>> > +               /*
>> > +                * The max utilization is always allowed for both clamps.
>> > +                * This is required to not force a null minimum utiliation on
>> > +                * all child groups.
>> > +                */
>> > +               uc_se->group_id = UCLAMP_NOT_VALID;
>> > +               uclamp_group_get(NULL, clamp_id, 0, uc_se,
>> > +                                uclamp_none(UCLAMP_MAX));
>>
>> I don't quite get why you are using uclamp_none(UCLAMP_MAX) for both
>> UCLAMP_MIN and UCLAMP_MAX clamps. I assume the comment above is to
>> explain this but I'm still unclear why this is done.
>
> That's maybe a bit tricky to get but, this will not happen since for
> root group tasks we apply the system default values... which however
> are introduced by one of the following patches 11/16.
>
> So, my understanding of the "delegation model" is that for cgroups we
> have to ensure each TG is a "restriction" of its parent. Thus:
>
>      tg::util_min <= tg_parent::util_min
>
> This is required to ensure that a tg_parent can always restrict
> resources on its descendants. I guess that's required to have a sane
> usage of CGroups for VMs where the Host can transparently control its
> Guests.
>
> According to the above rule, and considering that root task group
> cannot be modified, to allow boosting on TG we are forced to set the
> root group with util_min = SCHED_CAPACITY_SCALE.
>
> Moreover, Tejun pointed out that if we need tuning at root TG level,
> it means that we need system wide tunable, which should be available
> also when CGroups are not in use.
>
> That's why on patch:
>
>    [PATCH v4 11/16] sched/core: uclamp: add system default clamps
>    https://lore.kernel.org/lkml/20180828135324.21976-12-patrick.bellasi@arm.com/
>
> we add the concept of system default clamps which are actually
> initialized with util_min=0, i.e. 0% boost by default.
>
> These system default clamp values applies to tasks which are running
> either in the root task group on in an autogroup, which also cannot be
> tuned at run-time, whenever the task has not a task specific clamp
> value specified.
>
> All that considered, the code above is still confusing and I should
> consider moving to patch 11/16 the initialization to UCLAMP_MAX for
> util_min...
>
>> Maybe expand the comment to explain the intention?
>
> ... and add there something like:
>
>         /*
>          * The max utilization is always allowed for both clamps.
>          * This satisfies the "delegation model" required by CGroups
>          * v2, where a child task group cannot have more resources then
>          * its father, thus allowing the creation of child groups with
>          * a non null util_min.
>          * For tasks within the root_task_group we will use the system
>          * default clamp values anyway, thus they will not be boosted
>          * to the max utilization by default.
>          */
>
> It this more clear ?

Yes, I think so. Thanks for covering that.

>
>
>> With this I think all TGs will get boosted by default, won't they?
>
> You right, at cgroup creation time we clone parent's clamps... thus,
> all root_task_group's children group will get max boosting at creation
> time. However, since we don't have task within a newly created task
> group, the system management software can still refine the clamps
> before staring to move tasks in there.
>
> Do you think we should initialize root task group childrens
> differently ? I would prefer to avoid special cases if not strictly
> required...

I don't see a problem with the current approach.

>
> Cheers,
> Patrick
>
> --
> #include <best/regards.h>
>
> Patrick Bellasi

  reply index

Thread overview: 80+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-28 13:53 [PATCH v4 00/16] Add utilization clamping support Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 01/16] sched/core: uclamp: extend sched_setattr to support utilization clamping Patrick Bellasi
2018-09-05 11:01   ` Juri Lelli
2018-08-28 13:53 ` [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups Patrick Bellasi
2018-09-05 10:45   ` Juri Lelli
2018-09-06 13:48     ` Patrick Bellasi
2018-09-06 14:13       ` Juri Lelli
2018-09-06  8:17   ` Juri Lelli
2018-09-06 14:00     ` Patrick Bellasi
2018-09-08 23:47   ` Suren Baghdasaryan
2018-09-12 10:32     ` Patrick Bellasi
2018-09-12 13:49   ` Peter Zijlstra
2018-09-12 15:56     ` Patrick Bellasi
2018-09-12 16:12       ` Peter Zijlstra
2018-09-12 17:35         ` Patrick Bellasi
2018-09-12 17:42           ` Peter Zijlstra
2018-09-12 17:52             ` Patrick Bellasi
2018-09-13 19:14               ` Peter Zijlstra
2018-09-14  8:51                 ` Patrick Bellasi
2018-09-12 16:24   ` Peter Zijlstra
2018-09-12 17:42     ` Patrick Bellasi
2018-09-13 19:20       ` Peter Zijlstra
2018-09-14  8:47         ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 03/16] sched/core: uclamp: add CPU's clamp groups accounting Patrick Bellasi
2018-09-12 17:34   ` Peter Zijlstra
2018-09-12 17:44     ` Patrick Bellasi
2018-09-13 19:12   ` Peter Zijlstra
2018-09-14  9:07     ` Patrick Bellasi
2018-09-14 11:52       ` Peter Zijlstra
2018-09-14 13:41         ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 04/16] sched/core: uclamp: update CPU's refcount on clamp changes Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 05/16] sched/core: uclamp: enforce last task UCLAMP_MAX Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 06/16] sched/cpufreq: uclamp: add utilization clamping for FAIR tasks Patrick Bellasi
2018-09-14  9:32   ` Peter Zijlstra
2018-09-14 13:19     ` Patrick Bellasi
2018-09-14 13:36       ` Peter Zijlstra
2018-09-14 13:57         ` Patrick Bellasi
2018-09-27 10:23           ` Quentin Perret
2018-08-28 13:53 ` [PATCH v4 07/16] sched/core: uclamp: extend cpu's cgroup controller Patrick Bellasi
2018-08-28 18:29   ` Randy Dunlap
2018-08-29  8:53     ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 08/16] sched/core: uclamp: propagate parent clamps Patrick Bellasi
2018-09-09  3:02   ` Suren Baghdasaryan
2018-09-12 12:51     ` Patrick Bellasi
2018-09-12 15:56       ` Suren Baghdasaryan
2018-09-11 15:18   ` Tejun Heo
2018-09-11 16:26     ` Patrick Bellasi
2018-09-11 16:28       ` Tejun Heo
2018-08-28 13:53 ` [PATCH v4 09/16] sched/core: uclamp: map TG's clamp values into CPU's clamp groups Patrick Bellasi
2018-09-09 18:52   ` Suren Baghdasaryan
2018-09-12 14:19     ` Patrick Bellasi
2018-09-12 15:53       ` Suren Baghdasaryan [this message]
2018-08-28 13:53 ` [PATCH v4 10/16] sched/core: uclamp: use TG's clamps to restrict Task's clamps Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 11/16] sched/core: uclamp: add system default clamps Patrick Bellasi
2018-09-10 16:20   ` Suren Baghdasaryan
2018-09-11 16:46     ` Patrick Bellasi
2018-09-11 19:25       ` Suren Baghdasaryan
2018-08-28 13:53 ` [PATCH v4 12/16] sched/core: uclamp: update CPU's refcount on TG's clamp changes Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 13/16] sched/core: uclamp: use percentage clamp values Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 14/16] sched/core: uclamp: request CAP_SYS_ADMIN by default Patrick Bellasi
2018-09-04 13:47   ` Juri Lelli
2018-09-06 14:40     ` Patrick Bellasi
2018-09-06 14:59       ` Juri Lelli
2018-09-06 17:21         ` Patrick Bellasi
2018-09-14 11:10       ` Peter Zijlstra
2018-09-14 14:07         ` Patrick Bellasi
2018-09-14 14:28           ` Peter Zijlstra
2018-09-17 12:27             ` Patrick Bellasi
2018-09-21  9:13               ` Peter Zijlstra
2018-09-24 15:14                 ` Patrick Bellasi
2018-09-24 15:56                   ` Peter Zijlstra
2018-09-24 17:23                     ` Patrick Bellasi
2018-09-24 16:26                   ` Peter Zijlstra
2018-09-24 17:19                     ` Patrick Bellasi
2018-09-25 15:49                   ` Peter Zijlstra
2018-09-26 10:43                     ` Patrick Bellasi
2018-09-27 10:00                     ` Quentin Perret
2018-09-26 17:51                 ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 15/16] sched/core: uclamp: add clamp group discretization support Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 16/16] sched/cpufreq: uclamp: add utilization clamping for RT tasks Patrick Bellasi

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