linux-kernel - Re: [PATCH 7/7 v3] sched: fix wrong utilization accounting when switching to fair class

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Date:   Fri, 16 Sep 2016 14:45:11 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Yuyang Du <yuyang.du@...el.com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Paul Turner <pjt@...gle.com>,
        Benjamin Segall <bsegall@...gle.com>
Subject: Re: [PATCH 7/7 v3] sched: fix wrong utilization accounting when
 switching to fair class

On 16 September 2016 at 12:51, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Sep 12, 2016 at 09:47:52AM +0200, Vincent Guittot wrote:
>
> > -dequeue task
> > -put task
> > -change the property
> > -enqueue task
> > -set task as current task
>
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 3e52d08..7a9c9b9 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1105,10 +1105,10 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
> >
> >       p->sched_class->set_cpus_allowed(p, new_mask);
> >
> > -     if (running)
> > -             p->sched_class->set_curr_task(rq);
> >       if (queued)
> >               enqueue_task(rq, p, ENQUEUE_RESTORE);
> > +     if (running)
> > +             p->sched_class->set_curr_task(rq);
> >  }
> >
> >  /*
>
> So one thing that I've wanted to do for a while, but never managed to
> come up with a sensible way to do is encapsulate this pattern.
>
> The two options I came up with are:
>
> #define FOO(p, stmt)
> ({
>         struct rq *rq = task_rq(p);
>         bool queued = task_on_rq_queued(p);
>         bool running = task_current(rq);
>         int queue_flags = DEQUEUE_SAVE; /* also ENQUEUE_RESTORE */
>
>         if (queued)
>                 dequeue_task(rq, p, queue_flags);
>         if (running)
>                 put_prev_task(rq, p);
>
>         stmt;
>
>         if (queued)
>                 enqueue_task(rq, p, queue_flags);
>         if (running)
>                 set_curr_task(rq, p);
> })
>
> and
>
> void foo(struct task_struct *p, void (*func)(struct task_struct *, int *))
> {
>         struct rq *rq = task_rq(p);
>         bool queued = task_on_rq_queued(p);
>         bool running = task_current(rq);
>         int queue_flags = DEQUEUE_SAVE; /* also ENQUEUE_RESTORE */
>
>         if (queued)
>                 dequeue_task(rq, p, queue_flags);
>         if (running)
>                 put_prev_task(rq, p);
>
>         func(p, &queue_flags);
>
>         if (queued)
>                 enqueue_task(rq, p, queue_flags);
>         if (running)
>                 set_curr_task(rq, p);
> }
>
> Neither results in particularly pretty code. Although I suppose if I'd
> have to pick one I'd go for the macro variant.
>
> Opinions? I'm fine with leaving the code as is, just wanted to throw
> this out there.

I'm not convinced by using such encapsulation as it adds the
constraint of having a function to pass which is not always the case
and it hides a bit whats happen to this function
What about creating a task_FOO_save and a task_FOO_save macro  ? something like

#define task_FOO_save(p, rq, flags)
({
        bool queued = task_on_rq_queued(p);
        bool running = task_current(rq);
        int queue_flags = DEQUEUE_SAVE; /* also ENQUEUE_RESTORE */

        if (queued)
                dequeue_task(rq, p, queue_flags);
        if (running)
                put_prev_task(rq, p);
        flags = queued | running << 1;
})

#define task_FOO_restore(p, rq, flags)
({
        bool queued = flags & 0x1;
        bool running = flags & 0x2;
        int queue_flags = ENQUEUE_RESTORE;

        if (queued)
                enqueue_task(rq, p, queue_flags);
        if (running)
                set_curr_task(rq, p);
})