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

[Julia Lawall <julia.lawall@inria.fr>]

On Sat, 28 Oct 2023, Chen Yu wrote:

> On 2023-10-27 at 19:17:43 +0200, 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@inria.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;
> >  	}
> >
> > -	if (idle_smt == env->dst_cpu)
> > -		return true;
> > +	/* Is there an 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;
>
> Looks reasonable to me, if there is other idle SMT(from half-busy core)
> in the system, we should leverage that SMT to do the periodic lb.
> Per my understanding,

That's not the goal of this patch.  The goal of this patch is to avoid
doing return group_balance_cpu(sg) == env->dst_cpu; when a half-busy core
has been identified that is different from env->dst_cpu.

julia

>
> Reviewed-by: Chen Yu <yu.c.chen@intel.com>
>
> thanks,
> Chenyu
>