Re: [PATCH v2] sched/fair: Fix the decision for load balance

[Chen Yu]

On 2023-10-30 at 18:29:46 +0100, 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 | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2048138ce54b..69d63fae34f4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11079,12 +11079,16 @@ static int should_we_balance(struct lb_env *env)
>  			continue;
>  		}
>  
> -		/* Are we the first idle CPU? */
> +		/*
> +		 * Are we the first idle core in a MC or higher domain

It is possible that the Cluster domain is lower than a MC.
cluser domain: CPUs share the same L2
MC domain: CPUs share the same LLC

 grep . domain*/{name,flags}
domain0/name:CLS
domain1/name:MC
domain2/name:NUMA
domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING 
domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING 
domain2/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA 

So, maybe:
Are we the first idle core in a non-SMT domain or higher,

thanks,
Chenyu

> +		 * or the first idle CPU in a SMT domain?
> +		 */
>  		return cpu == env->dst_cpu;
>  	}
>  
> -	if (idle_smt == env->dst_cpu)
> -		return true;
> +	/* 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;
> -- 
> 2.34.1
>