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

[Keisuke Nishimura]

On 30/10/2023 09:05, Vincent Guittot wrote:
> On Mon, 30 Oct 2023 at 05:03, Shrikanth Hegde
> <sshegde@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxx>
> > > ---
> > >   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

Maybe I should update the comment with the additional contexts:

/*
 * Are we the first idle core in a sched_domain not-sharing capacity,
 * or the first idle CPU in a sched_domain sharing capacity?
 */


> > > -     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 */

OK, agreed. Should I create version 2?

thanks,
Keisuke

> > > +     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@xxxxxxxxxxxxxxxxxx>