linux-kernel - Re: [PATCH v5 02/10] sched/rt: add rt

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Date:   Wed, 30 May 2018 12:06:37 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        viresh kumar <viresh.kumar@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Quentin Perret <quentin.perret@....com>
Subject: Re: [PATCH v5 02/10] sched/rt: add rt_rq utilization tracking

On 30 May 2018 at 11:32, Patrick Bellasi <patrick.bellasi@....com> wrote:
> On 29-May 15:29, Vincent Guittot wrote:
>> Hi Patrick,
>> >> +static inline bool rt_rq_has_blocked(struct rq *rq)
>> >> +{
>> >> +     if (rq->avg_rt.util_avg)
>> >
>> > Should use READ_ONCE?
>>
>> I was expecting that there will be only one read by default but I can
>> add READ_ONCE
>
> I would say here it's required mainly for "documentation" purposes,
> since we can use this function from non rq-locked paths, e.g.
>
>    update_sg_lb_stats()
>       update_nohz_stats()
>          update_blocked_averages()
>             rt_rq_has_blocked()
>
> Thus, AFAIU, we should use READ_ONCE to "flag" that the value can
> potentially be updated concurrently?

yes

>
>> >
>> >> +             return true;
>> >> +
>> >> +     return false;
>> >
>> > What about just:
>> >
>> >        return READ_ONCE(rq->avg_rt.util_avg);
>> >
>> > ?
>>
>> This function is renamed and extended with others tracking in the
>> following patches so we have to test several values in the function.
>> That's also why there is the if test because additional if test are
>> going to be added
>
> Right, makes sense.
>
> [...]
>
>> >> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>> >> index ef3c4e6..b4148a9 100644
>> >> --- a/kernel/sched/rt.c
>> >> +++ b/kernel/sched/rt.c
>> >> @@ -5,6 +5,8 @@
>> >>   */
>> >>  #include "sched.h"
>> >>
>> >> +#include "pelt.h"
>> >> +
>> >>  int sched_rr_timeslice = RR_TIMESLICE;
>> >>  int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
>> >>
>> >> @@ -1572,6 +1574,9 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>> >>
>> >>       rt_queue_push_tasks(rq);
>> >>
>> >> +     update_rt_rq_load_avg(rq_clock_task(rq), rq,
>> >> +             rq->curr->sched_class == &rt_sched_class);
>> >> +
>> >>       return p;
>> >>  }
>> >>
>> >> @@ -1579,6 +1584,8 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
>> >>  {
>> >>       update_curr_rt(rq);
>> >>
>> >> +     update_rt_rq_load_avg(rq_clock_task(rq), rq, 1);
>> >> +
>> >>       /*
>> >>        * The previous task needs to be made eligible for pushing
>> >>        * if it is still active
>> >> @@ -2308,6 +2315,7 @@ static void task_tick_rt(struct rq *rq, struct task_struct *p, int queued)
>> >>       struct sched_rt_entity *rt_se = &p->rt;
>> >>
>> >>       update_curr_rt(rq);
>> >> +     update_rt_rq_load_avg(rq_clock_task(rq), rq, 1);
>> >
>> > Mmm... not entirely sure... can't we fold
>> >    update_rt_rq_load_avg() into update_curr_rt() ?
>> >
>> > Currently update_curr_rt() is used in:
>> >    dequeue_task_rt
>> >    pick_next_task_rt
>> >    put_prev_task_rt
>> >    task_tick_rt
>> >
>> > while we update_rt_rq_load_avg() only in:
>> >    pick_next_task_rt
>> >    put_prev_task_rt
>> >    task_tick_rt
>> > and
>> >    update_blocked_averages
>> >
>> > Why we don't we need to update at dequeue_task_rt() time ?
>>
>> We are tracking rt rq and not sched entities so we want to know when
>> sched rt will be the running or not  sched class on the rq. Tracking
>> dequeue_task_rt is useless
>
> What about (push) migrations?

it doesn't make any difference. put_prev_task_rt() says that the prev
task that was running, was a rt task so we can account past time at rt
running time
and pick_next_task_rt says that the next one will be a rt task so we
have to account elapse time either to rt or not rt time according.

I can probably optimize the pick_next_task_rt by doing the below instead:

if (rq->curr->sched_class == &rt_sched_class)
       update_rt_rq_load_avg(rq_clock_task(rq), rq, 0);

If prev task is a rt  task, put_prev_task_rt has already done the update

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