linux-kernel - Re: [PATCH] fix scheduler regression from "sched/fair: Rework load

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Date:   Mon, 26 Oct 2020 10:38:23 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Chris Mason <clm@...com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fix scheduler regression from "sched/fair: Rework
 load_balance()"

On Mon, 2020-10-26 at 15:24 +0100, 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@...com> 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.
> 
> 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.

Should we also spread tasks when balancing between
multi-threaded CPU cores on the same socket?

Say we have groups of CPUs
(0, 2) and CPUs (1, 3),
with CPU 2 idle, and 3 tasks spread between CPUs
1 & 3.

Since they are all on the same LLC, and the task
wakeup code has absolutely no hesitation in moving
them around, should the load balancer also try to
keep tasks within a socket spread across all CPUs?

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