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

[Vincent Guittot]

On Tue, 11 May 2021 at 19:25, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 5/11/21 8:25 AM, Vincent Guittot wrote:
> > Hi Tim,
> >
> > Sometimes, we want to set this_rq->next_balance backward compared to
> > its current value. When a CPU is busy, the balance interval is
> > multiplied by busy_factor which is set to 16 by default. On SMT2 sched
> > domain level, it means that the interval will be 32ms when busy
> > instead of 2ms. But if a busy load balance happens just before
> > becoming idle, the this_rq->next_balance will be set 32ms later
> > whereas it should go down to 2ms as the CPU is now idle. And this
> > becomes even worse when you have 128 CPUs at die sched_domain level
> > because the idle CPU will have to wait 2048ms instead of the correct
> > 128ms interval.
> >
> >>
> >>  out:
> >>         /* Move the next balance forward */
> >> -       if (time_after(this_rq->next_balance, next_balance))
> >> +       if (time_after(next_balance, this_rq->next_balance))
> >
> > The current comparison is correct but next_balance should not be in the past.
>
> I understand then the intention is after the update,
> this_rq->next_balance should have a minimum value of jiffies+1. So
> we will need
>
>  out:
>         /* Move the next balance forward */
> +       this_rq->next_balance = max(jiffies+1, this_rq->next_balance);
>         if (time_after(this_rq->next_balance, next_balance))
>                 this_rq->next_balance = next_balance;
>
> as next_balance computed will be at least jiffies+1 after your fix to
> update_next_balance(). We still need to take care of the case when
> this_rq->next_balance <= jiffies.
>
> So combining with your suggestion on the fix to update_next_balance(),
> the fix will be
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1d75af1ecfb4..2dc471c1511c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9901,7 +9901,7 @@ update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
>
>         /* used by idle balance, so cpu_busy = 0 */
>         interval = get_sd_balance_interval(sd, 0);
> -       next = sd->last_balance + interval;
> +       next = max(jiffies+1, sd->last_balance + interval);
>
>         if (time_after(*next_balance, next))
>                 *next_balance = next;
> @@ -10681,6 +10681,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>
>  out:
>         /* Move the next balance forward */
> +       this_rq->next_balance = max(jiffies+1, this_rq->next_balance);
>         if (time_after(this_rq->next_balance, next_balance))
>                 this_rq->next_balance = next_balance;
>
>
> >
> > update_next_balance() is only used in newidle_balance() so we could
> > modify it to  have:
> >
> > next = max(jiffies+1, next = sd->last_balance + interval)
>
> Is the extra assignment "next = sd->last_balance + interval" needed?

No it's a typo mistake while copy pasting the line

> This seems more straight forward:
>
> next = max(jiffies+1, sd->last_balance + interval)
>
> I will try to get the benchmark folks to do another run with this change.
> Hopefully I'll get some bandwidth from them soon.
>
> Thanks.
>
> Tim
>