Re: [PATCH] sched/cache: Check reduced capacity before alb_break_llc()

[Kayra Cizmeci]

On Thu, Oct 29, 2026 at 10:36:50AM +0800, Zhan Xusheng wrote:

Hi Zhan,

> From: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> 
> need_active_balance() moves the only CFS task off a CPU whose capacity
> is reduced, for example by IRQ load, to an idle CPU with more.
> alb_break_llc() runs first and refuses to move a lone task off its
> preferred LLC, so if no other CPU in that LLC is idle the task stays on
> the reduced CPU.  Like asym packing, which was moved ahead of
> alb_break_llc() for the same reason, this is a performance case that
> cache locality should not override.
> 
> With two LLCs of two CPUs, the other CPU of the task's LLC busy and 60%
> of the task's CPU spent in a hardirq, a CPU-bound two-thread process
> stayed there for 20 seconds at 38% of its normal throughput.  With this
> change it moves to the idle LLC within a second, as it does with cache
> aware scheduling disabled.

Sweet.

> 
> Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from their preferred LLC")
> Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cfc982205b98..e313c6d212c6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -13585,12 +13585,6 @@ static int need_active_balance(struct lb_env *env)
>  	if (asym_active_balance(env))
>  		return 1;
>  
> -	if (alb_break_llc(env))
> -		return 0;
> -
> -	if (imbalanced_active_balance(env))
> -		return 1;
> -
>  	/*
>  	 * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
>  	 * It's worth migrating the task if the src_cpu's capacity is reduced
> @@ -13604,6 +13598,12 @@ static int need_active_balance(struct lb_env *env)
>  			return 1;
>  	}
>  
> +	if (alb_break_llc(env))
> +		return 0;
> +
> +	if (imbalanced_active_balance(env))
> +		return 1;
> +
>  	if (env->migration_type == migrate_misfit ||
>  	    env->migration_type == migrate_llc_task)
>  		return 1;
> 
> base-commit: 65ba8f34ec886ec72b2c07ddc8cf3d815c32869d
> -- 
> 2.43.0
> 

Well yeah. alb_break_llc() just returns true in the nr_running <= 1
case. FWIW, on my tip tree this just creates a conflict. On me
alb_break_llc() is first, then asym_active_balance() and then
imbalanced_active_balance(). I think in that case moving all three
functions would be better? (below this other block that's for this case)



Thanks,

Kayra