Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ

[Vincent Guittot]

Hi Tim,

On Wed, 24 Mar 2021 at 17:05, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 3/24/21 6:44 AM, Vincent Guittot wrote:
> > Hi Tim,
>
> >
> > IIUC your problem, we call update_blocked_averages() but because of:
> >
> >               if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost) {
> >                       update_next_balance(sd, &next_balance);
> >                       break;
> >               }
> >
> > the for_each_domain loop stops even before running load_balance on the 1st
> > sched domain level which means that update_blocked_averages() was called
> > unnecessarily.
> >
>
> That's right
>
> > And this is even more true with a small sysctl_sched_migration_cost which allows newly
> > idle LB for very small this_rq->avg_idle. We could wonder why you set such a low value
> > for sysctl_sched_migration_cost which is lower than the max_newidle_lb_cost of the
> > smallest domain but that's probably because of task_hot().
> >
> > if avg_idle is lower than the sd->max_newidle_lb_cost of the 1st sched_domain, we should
> > skip spin_unlock/lock and for_each_domain() loop entirely
> >
> > Maybe something like below:
> >
>
> The patch makes sense.  I'll ask our benchmark team to queue this patch for testing.

Do you have feedback from your benchmark team ?

Regards,
Vincent
>
> Tim
>
>