Re: [PATCH] fix scheduler regression from "sched/fair: Rework load_balance()"

[Chris Mason]

On 26 Oct 2020, at 12:20, Vincent Guittot wrote:

> Le lundi 26 oct. 2020 à 12:04:45 (-0400), Rik van Riel a écrit :
> > On Mon, 26 Oct 2020 16:42:14 +0100
> > Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> > > On Mon, 26 Oct 2020 at 16:04, Rik van Riel <riel@xxxxxxxxxxx> wrote:
> >
> > > > Could utilization estimates be off, either lagging or
> > > > simply having a wrong estimate for a task, resulting
> > > > in no task getting pulled sometimes, while doing a
> > > > migrate_task imbalance always moves over something?
> > >
> > > task and cpu utilization are not always up to fully synced and may 
> > > lag
> > > a bit which explains that sometimes LB can fail to migrate for a 
> > > small
> > > diff
> >
> > OK, running with this little snippet below, I see latencies
> > improve back to near where they used to be:
> >
> > Latency percentiles (usec) runtime 150 (s)
> >         50.0th: 13
> >         75.0th: 31
> >         90.0th: 69
> >         95.0th: 90
> >         *99.0th: 761
> >         99.5th: 2268
> >         99.9th: 9104
> >         min=1, max=16158
> >
> > I suspect the right/cleaner approach might be to use
> > migrate_task more in !CPU_NOT_IDLE cases?
> >
> > Running a task to an idle CPU immediately, instead of refusing
> > to have the load balancer move it, improves latencies for fairly
> > obvious reasons.
> >
> > I am not entirely clear on why the load balancer should need to
> > be any more conservative about moving tasks than the wakeup
> > path is in eg. select_idle_sibling.
>
>
> what you are suggesting is something like:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4978964e75e5..3b6fbf33abc2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9156,7 +9156,8 @@ static inline void calculate_imbalance(struct 
> lb_env *env, struct sd_lb_stats *s
>          * emptying busiest.
>          */
>         if (local->group_type == group_has_spare) {
> -               if (busiest->group_type > group_fully_busy) {
> +               if ((busiest->group_type > group_fully_busy) &&
> +                   !(env->sd->flags & SD_SHARE_PKG_RESOURCES)) {
>                         /*
>                          * If busiest is overloaded, try to fill spare
>                          * capacity. This might end up creating spare 
> capacity
>
> which also fixes the problem for me and alignes LB with wakeup path 
> regarding the migration
> in the LLC

Vincent’s patch on top of 5.10-rc1 looks pretty great:

Latency percentiles (usec) runtime 90 (s) (3320 total samples)
        50.0th: 161 (1687 samples)
        75.0th: 200 (817 samples)
        90.0th: 228 (488 samples)
        95.0th: 254 (164 samples)
        *99.0th: 314 (131 samples)
        99.5th: 330 (17 samples)
        99.9th: 356 (13 samples)
        min=29, max=358

Next we test in prod, which probably won’t have answers until 
tomorrow.  Thanks again Vincent!

-chris