linux-kernel - Re: [PATCH v8 08/16] sched/core: uclamp: Set default clamps for RT tasks

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Date:   Tue, 7 May 2019 12:25:22 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH v8 08/16] sched/core: uclamp: Set default clamps for RT
 tasks

On 17-Apr 16:07, Suren Baghdasaryan wrote:
> On Tue, Apr 2, 2019 at 3:42 AM Patrick Bellasi <patrick.bellasi@....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@....com>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Peter Zijlstra <peterz@...radead.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.
> 

The util_max for an RT task can be less then uclamp_none(UCLAMP_MAX),
however, requesting a task specific util_max which is smaller then the
current util_min will fail in:

   __sched_setscheduler()
      uclamp_validate()

since we only allow util_min <= util_max for all task specific values.

> > +       }
> > +
> >         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
> >

-- 
#include <best/regards.h>

Patrick Bellasi