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

[Chris Mason]

On 26 Oct 2020, at 10:24, Vincent Guittot wrote:

> Le lundi 26 oct. 2020 à 08:45:27 (-0400), Chris Mason a écrit :
> > On 26 Oct 2020, at 4:39, Vincent Guittot wrote:
> >
> > > Hi Chris
> > >
> > > On Sat, 24 Oct 2020 at 01:49, Chris Mason <clm@xxxxxx> wrote:
> > > > Hi everyone,
> > > >
> > > > We’re validating a new kernel in the fleet, and compared with 
> > > > v5.2,
> > >
> > > Which version are you using ?
> > > several improvements have been added since v5.5 and the rework of
> > > load_balance
> >
> > We’re validating v5.6, but all of the numbers referenced in this 
> > patch are
> > against v5.9.  I usually try to back port my way to victory on this 
> > kind of
> > thing, but mainline seems to behave exactly the same as 0b0695f2b34a 
> > wrt
> > this benchmark.
>
> ok. Thanks for the confirmation
>
> I have been able to reproduce the problem on my setup.

Thanks for taking a look!  Can I ask what parameters you used on 
schbench, and what kind of results you saw?  Mostly I’m trying to make 
sure it’s a useful tool, but also the patch didn’t change things 
here.

> Could you try the fix below ?
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9049,7 +9049,8 @@ 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) &&
> +                   (busiest->group_weight > 1)) {
>                         /*
>                          * If busiest is overloaded, try to fill spare
>                          * capacity. This might end up creating spare 
> capacity
>
>
> When we calculate an imbalance at te smallest level, ie between CPUs 
> (group_weight == 1),
> we should try to spread tasks on cpus instead of trying to fill spare 
> capacity.

With this patch on top of v5.9, my latencies are unchanged.  I’m 
building against current Linus now just in case I’m missing other 
fixes.

-chris