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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Mon, 21 Jun 2021 10:38:36 +0200
Message-ID: <CAKfTPtDgLRYdFNY_A3=yXk-LXi-eH21xApAaYfYk_RbG4k61EQ@mail.gmail.com>
Subject: Re: [PATCH 1/2] sched/rt: Fix RT utilization tracking during policy change
To:     Vincent Donnefort <vincent.donnefort@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Fri, 18 Jun 2021 at 15:32, Vincent Donnefort
<vincent.donnefort@arm.com> wrote:
>
> RT keeps track of the utilization on a per-rq basis with the structure
> avg_rt. This utilization is updated during task_tick_rt(),
> put_prev_task_rt() and set_next_task_rt(). However, when the current
> running task changes its policy, set_next_task_rt() which would usually
> take care of updating the utilization when the rq starts running RT tasks,
> will not see a such change, leaving the avg_rt structure outdated. When
> that very same task will be dequeued later, put_prev_task_rt() will then
> update the utilization, based on a wrong last_update_time, leading a huge
> spike in the RT utilization signal.
>
> The signal would eventually recover from this issue after few ms. Even if
> no RT tasks are run, avg_rt is also updated in __update_blocked_others().
> But as the CPU capacity depends partly on the avg_rt, this issue has
> nonetheless a significant impact on the scheduler.
>
> Fix this issue by ensuring a load update when a running task changes
> its policy to RT.
>
> Fixes: 371bf427 ("sched/rt: Add rt_rq utilization tracking")
> Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index a525447..5ac0f31 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2341,10 +2341,8 @@ void __init init_sched_rt_class(void)
>  static void switched_to_rt(struct rq *rq, struct task_struct *p)
>  {
>         /*
> -        * If we are already running, then there's nothing
> -        * that needs to be done. But if we are not running
> -        * we may need to preempt the current running task.
> -        * If that current running task is also an RT task
> +        * If we are not running we may need to preempt the current
> +        * running task. If that current running task is also an RT task
>          * then see if we can move to another run queue.
>          */
>         if (task_on_rq_queued(p) && rq->curr != p) {
> @@ -2355,6 +2353,13 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
>                 if (p->prio < rq->curr->prio && cpu_online(cpu_of(rq)))
>                         resched_curr(rq);
>         }
> +
> +       /*
> +        * If we are running, update the avg_rt tracking, as the running time
> +        * will now on be accounted into the latter.
> +        */
> +       if (task_current(rq, p))

task_current(rq,p) equals the "rq->curr != p" above. Having 2
different ways to get the same value in the function is a bit
confusing.

And call it once

Otherwise, the fix looks good to me

> +               update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 0);
>  }
>
>  /*
> --
> 2.7.4
>