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

[Tim Chen]

On Thu, 2026-10-29 at 10:36 +0800, Zhan Xusheng wrote:
> 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.  
> 

The patch moves the task without knowing whether another CPU in
the preferred LLC is idle.

> 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.

I agree that this is an issue.  But I have a couple of reservations about
the proposed fix.

1) There could be other idle CPUs in the preferred LLC. It would be
better for the task to move to one of them, through the idle balance
on those CPUs (via same reduced capacity check), than to move to a non-preferred LLC now.

I think the LLC override should apply only when no idle CPU in the
preferred LLC has enough capacity, as your change log describes.

2) There could be ping pong migrations.  Cache aware scheduling could 
move the task back to the CPU, because with only IRQ load on it, that
CPU is considered idle. I think this would also need to be dealt with.

Could you check nr_migrations in /proc/<pid>/sched, or trace
sched_migrate_task, over the 20 seconds on your setup
before and after your patch?  A run with 20% to 40% irq load would also help.

Tim

> 
> 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