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

[Chen Yu]

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@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;
>  	}
>  
> -	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,

Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>

thanks,
Chenyu