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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 17 Apr 2019 16:07:48 -0700
Message-ID: <CAJuCfpH9E2Khc1WP8MZgLK+yF7GgwW29ECES4hqPN3jaB34RqA@mail.gmail.com>
Subject: Re: [PATCH v8 08/16] sched/core: uclamp: Set default clamps for RT tasks
To:     Patrick Bellasi <patrick.bellasi@arm.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-pm@vger.kernel.org,
        linux-api@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>,
        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>
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On Tue, Apr 2, 2019 at 3:42 AM Patrick Bellasi <patrick.bellasi@arm.com> wrote:
>
> By default FAIR tasks start without clamps, i.e. neither boosted nor
> capped, and they run at the best frequency matching their utilization
> demand.  This default behavior does not fit RT tasks which instead are
> expected to run at the maximum available frequency, if not otherwise
> required by explicitly capping them.
>
> Enforce the correct behavior for RT tasks by setting util_min to max
> whenever:
>
>  1. the task is switched to the RT class and it does not already have a
>     user-defined clamp value assigned.
>
>  2. an RT task is forked from a parent with RESET_ON_FORK set.
>
> NOTE: utilization clamp values are cross scheduling class attributes and
> thus they are never changed/reset once a value has been explicitly
> defined from user-space.
>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> ---
>  kernel/sched/core.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bdebdabe9bc4..71c9dd6487b1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1042,6 +1042,28 @@ static int uclamp_validate(struct task_struct *p,
>  static void __setscheduler_uclamp(struct task_struct *p,
>                                   const struct sched_attr *attr)
>  {
> +       unsigned int clamp_id;
> +
> +       /*
> +        * On scheduling class change, reset to default clamps for tasks
> +        * without a task-specific value.
> +        */
> +       for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
> +               struct uclamp_se *uc_se = &p->uclamp_req[clamp_id];
> +               unsigned int clamp_value = uclamp_none(clamp_id);
> +
> +               /* Keep using defined clamps across class changes */
> +               if (uc_se->user_defined)
> +                       continue;
> +
> +               /* By default, RT tasks always get 100% boost */
> +               if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> +                       clamp_value = uclamp_none(UCLAMP_MAX);
> +
> +               uc_se->bucket_id = uclamp_bucket_id(clamp_value);
> +               uc_se->value = clamp_value;

Is it possible for p->uclamp_req[UCLAMP_MAX].value to be less than
uclamp_none(UCLAMP_MAX) for this RT task? If that's a possibility then
I think we will end up with a case of p->uclamp_req[UCLAMP_MIN].value
> p->uclamp_req[UCLAMP_MAX].value after these assignments are done.

> +       }
> +
>         if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
>                 return;
>
> @@ -1077,6 +1099,10 @@ static void uclamp_fork(struct task_struct *p)
>         for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
>                 unsigned int clamp_value = uclamp_none(clamp_id);
>
> +               /* By default, RT tasks always get 100% boost */
> +               if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> +                       clamp_value = uclamp_none(UCLAMP_MAX);
> +
>                 p->uclamp_req[clamp_id].user_defined = false;
>                 p->uclamp_req[clamp_id].value = clamp_value;
>                 p->uclamp_req[clamp_id].bucket_id = uclamp_bucket_id(clamp_value);
> --
> 2.20.1
>