linux-kernel - Re: [PATCH] sched/core: Fix reset-on-fork from RT with uclamp

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Date:   Wed, 15 Apr 2020 09:20:01 +0100
From:   Quentin Perret <qperret@...gle.com>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, juri.lelli@...hat.com,
        Vincent Guittot <vincent.guittot@...aro.org>,
        dietmar.eggemann@....com, Steven Rostedt <rostedt@...dmis.org>,
        bsegall@...gle.com, mgorman@...e.de, ctheegal@...eaurora.org,
        patrick.bellasi@...bug.net, valentin.schneider@....com,
        qais.yousef@....com, LKML <linux-kernel@...r.kernel.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH] sched/core: Fix reset-on-fork from RT with uclamp

Hi Doug,

On Tuesday 14 Apr 2020 at 13:45:03 (-0700), Doug Anderson wrote:
> Hi,
> 
> On Tue, Apr 14, 2020 at 9:13 AM Quentin Perret <qperret@...gle.com> wrote:
> >
> > uclamp_fork() resets the uclamp values to their default when the
> > reset-on-fork flag is set. It also checks whether the task has a RT
> > policy, and sets its uclamp.min to 1024 accordingly. However, during
> > reset-on-fork, the task's policy is lowered to SCHED_NORMAL right after,
> > hence leading to an erroneous uclamp.min setting for the new task if it
> > was forked from RT.
> >
> > Fix this by removing the unnecessary check on rt_policy() in
> > uclamp_fork() as this doesn't make sense if the reset-on-fork flag is
> > set.
> >
> > Reported-by: Chitti Babu Theegala <ctheegal@...eaurora.org>
> > Signed-off-by: Quentin Perret <qperret@...gle.com>
> > ---
> >  kernel/sched/core.c | 4 ----
> >  1 file changed, 4 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 3a61a3b8eaa9..9ea3e484eea2 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1234,10 +1234,6 @@ static void uclamp_fork(struct task_struct *p)
> >         for_each_clamp_id(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);
> > -
> >                 uclamp_se_set(&p->uclamp_req[clamp_id], clamp_value, false);
> 
> The local variable "clamp_value" doesn't have a lot of value anymore,
> does it?  (Pun intended).

:)

> Remove it?

Right, but I figured the generated code should be similar, and
'uclamp_se_set(&p->uclamp_req[clamp_id], uclamp_none(clamp_id), false);'
doesn't fit in 80 cols at this identation level, so I kept the local
var. No strong opinion, though.

Thanks,
Quentin