linux-kernel - Re: [PATCH] sched/core: forced idle accounting

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Date:   Mon, 11 Oct 2021 17:31:41 -0700
From:   Josh Don <joshdon@...gle.com>
To:     Hao Luo <haoluo@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Vineeth Pillai <vineethrp@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/core: forced idle accounting

On Mon, Oct 11, 2021 at 10:33 AM Hao Luo <haoluo@...gle.com> wrote:
>
> On Thu, Oct 7, 2021 at 5:08 PM Josh Don <joshdon@...gle.com> wrote:
> > -void sched_core_dequeue(struct rq *rq, struct task_struct *p)
> > +void sched_core_dequeue(struct rq *rq, struct task_struct *p, int flags)
> >  {
> >         rq->core->core_task_seq++;
> >
> > -       if (!sched_core_enqueued(p))
> > -               return;
> > +       if (sched_core_enqueued(p)) {
> > +               rb_erase(&p->core_node, &rq->core_tree);
> > +               RB_CLEAR_NODE(&p->core_node);
> > +       }
> >
> > -       rb_erase(&p->core_node, &rq->core_tree);
> > -       RB_CLEAR_NODE(&p->core_node);
> > +       /*
> > +        * Migrating the last task off the cpu, with the cpu in forced idle
> > +        * state. Reschedule to create an accounting edge for forced idle,
> > +        * and re-examine whether the core is still in forced idle state.
> > +        */
> > +       if (!(flags & DEQUEUE_SAVE) && rq->nr_running == 1 &&
> > +           rq->core->core_forceidle && rq->curr == rq->idle)
> > +               resched_curr(rq);
>
> Resched_curr is probably an unwanted side effect of dequeue. Maybe we
> could extract the check and resched_curr out into a function, and call
> the function outside of sched_core_dequeue(). In that way, the
> interface of dequeue doesn't need to change.

This resched is an atypical case; normal load balancing won't steal
the last runnable task off a cpu. The main reasons this resched could
trigger are: migration due to affinity change, and migration due to
sched core doing a cookie_steal. Could bubble this up to
deactivate_task(), but seems less brittle to keep this in dequeue()
with the check against DEQUEUE_SAVE (since this creates an important
accounting edge). Thoughts?

> >         /*
> > @@ -5765,7 +5782,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >         for_each_cpu_wrap(i, smt_mask, cpu) {
> >                 rq_i = cpu_rq(i);
> >
> > -               if (i != cpu)
> > +               if (i != cpu && (rq_i != rq->core || !core_clock_updated))
> >                         update_rq_clock(rq_i);
>
> Do you mean (rq_i != rq->core && !core_clock_updated)? I thought
> rq->core has core_clock updated always.

rq->clock is updated on entry to pick_next_task(). rq->core is only
updated if rq == rq->core, or if we've done the clock update for
rq->core above.