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SubjectRe: [PATCH 1/6] sched/fair: remove update of blocked load from newidle_balance
On Tue, 9 Feb 2021 at 14:09, Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
> On 05/02/21 12:48, Vincent Guittot wrote:
> > @@ -10517,16 +10499,11 @@ static void nohz_newidle_balance(struct rq *this_rq)
> > time_before(jiffies, READ_ONCE(nohz.next_blocked)))
> > return;
> >
>
> I was wondering whether all the conditions above were still relevant. I
> think they are, but this one:
>
> /* Will wake up very soon. No time for doing anything else*/
> if (this_rq->avg_idle < sysctl_sched_migration_cost)
> return;
>
> should have its comment updated to something like:
>
> /*
> * Will wake up very soon. Blocked load will be updated
> * periodically, no need to wake an idle CPU.
> */
>
> given kick_ilb() isn't the costliest of things.
>
> > - raw_spin_unlock(&this_rq->lock);
> > /*
> > - * This CPU is going to be idle and blocked load of idle CPUs
> > - * need to be updated. Run the ilb locally as it is a good
> > - * candidate for ilb instead of waking up another idle CPU.
> > - * Kick an normal ilb if we failed to do the update.
> > + * Blocked load of idle CPUs need to be updated.
> > + * Kick an ILB to update statistics.
> > */
> > - if (!_nohz_idle_balance(this_rq, NOHZ_STATS_KICK, CPU_NEWLY_IDLE))
> > - kick_ilb(NOHZ_STATS_KICK);
> > - raw_spin_lock(&this_rq->lock);
>
> With this change, the return value of _nohz_idle_balance() is no longer
> used. This means we could get rid of the tracking of whether it iterated
> over all nohz CPUs or not.

Yeah, the return is useless now

>
> > + kick_ilb(NOHZ_STATS_KICK);
> > }
> >
> > #else /* !CONFIG_NO_HZ_COMMON */

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