linux-kernel - Re: [PATCH] sched/fair: Fix the decision for load balance

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Date:   Mon, 30 Oct 2023 09:05:08 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
Cc:     Keisuke Nishimura <keisuke.nishimura@...ia.fr>,
        linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Julia Lawall <julia.lawall@...ia.fr>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] sched/fair: Fix the decision for load balance

On Mon, 30 Oct 2023 at 05:03, Shrikanth Hegde
<sshegde@...ux.vnet.ibm.com> wrote:
>
>
>
> On 10/27/23 10:47 PM, Keisuke Nishimura wrote:
> > should_we_balance is called for the decision to do load-balancing.
> > When sched ticks invoke this function, only one CPU should return
> > true. However, in the current code, two CPUs can return true. The
> > following situation, where b means busy and i means idle, is an
> > example because CPU 0 and CPU 2 return true.
> >
> >         [0, 1] [2, 3]
> >          b  b   i  b
> >
> > This fix checks if there exists an idle CPU with busy sibling(s)
> > after looking for a CPU on an idle core. If some idle CPUs with busy
> > siblings are found, just the first one should do load-balancing.
> >
>
> > Fixes: b1bfeab9b002 ("sched/fair: Consider the idle state of the whole core for load balance")
> > Signed-off-by: Keisuke Nishimura <keisuke.nishimura@...ia.fr>
> > ---
> >  kernel/sched/fair.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 2048138ce54b..eff0316d6c7d 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -11083,8 +11083,9 @@ static int should_we_balance(struct lb_env *env)
> >               return cpu == env->dst_cpu;
> >       }
> >
>
>
> There is comment above this /* Are we the first idle CPU? */
> Maybe update that comment as /* Are we the first idle core */

I was about to say the same but it's not always true. If we are at SMT
level, we look for an idle CPU in the core

>
> > -     if (idle_smt == env->dst_cpu)
> > -             return true;
> > +     /* Is there an idle CPU with busy siblings? */
> nit: We can keep the comment style fixed in this function.
> /* Are we the first idle CPU with busy siblings */
>
> > +     if (idle_smt != -1)
> > +             return idle_smt == env->dst_cpu;
> >
> >       /* Are we the first CPU of this group ? */
> >       return group_balance_cpu(sg) == env->dst_cpu;
>
> code changes LGTM
> Reviewed-by: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>