Re: [PATCH v2] sched/fair: Prefer cache locality for EAS wakeup

[Christian Loehle <christian.loehle@arm.com>]

On 10/30/25 19:19, Shubhang Kaushik via B4 Relay wrote:
> From: Shubhang Kaushik <shubhang@os.amperecomputing.com>
> 
> When Energy Aware Scheduling (EAS) is enabled, a task waking up on a
> sibling CPU might migrate away from its previous CPU even if that CPU
> is not overutilized. This sacrifices cache locality and introduces
> unnecessary migration overhead.
> 
> This patch refines the wakeup heuristic in `select_idle_sibling()`. If
> EAS is active and the task's previous CPU (`prev`) is not overutilized,
> the scheduler will prioritize waking the task on `prev`, avoiding an
> unneeded migration and preserving cache-hotness.
> 
> ---
> v2:
> - Addressed reviewer comments to handle this special condition
>   within the selection logic, prioritizing the
>   previous CPU if not overutilized for EAS.
> - Link to v1: https://lore.kernel.org/all/20251017-b4-sched-cfs-refactor-propagate-v1-1-1eb0dc5b19b3@os.amperecomputing.com/
> 
> Signed-off-by: Shubhang Kaushik <shubhang@os.amperecomputing.com>
> ---
>  kernel/sched/fair.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 25970dbbb27959bc130d288d5f80677f75f8db8b..ac94463627778f09522fb5420f67b903a694ad4d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7847,9 +7847,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  	    asym_fits_cpu(task_util, util_min, util_max, target))
>  		return target;
>  
> -	/*
> -	 * If the previous CPU is cache affine and idle, don't be stupid:
> -	 */
> +	/* Reschedule on an idle, cache-sharing sibling to preserve affinity: */
>  	if (prev != target && cpus_share_cache(prev, target) &&
>  	    (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
>  	    asym_fits_cpu(task_util, util_min, util_max, prev)) {
> @@ -7861,6 +7859,14 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  		prev_aff = prev;
>  	}
>  
> +	/*
> +	 * If the previous CPU is not overutilized, prefer it for cache locality.
> +	 * This prevents migration away from a cache-hot CPU that can still
> +	 * handle the task without causing an overload.
> +	 */
> +	if (sched_energy_enabled() && !cpu_overutilized(prev))
> +		return prev;
> +
>  	/*
>  	 * Allow a per-cpu kthread to stack with the wakee if the
>  	 * kworker thread and the tasks previous CPUs are the same.
> 
> ---
> base-commit: e53642b87a4f4b03a8d7e5f8507fc3cd0c595ea6
> change-id: 20251030-b4-follow-up-ff03b4533a2d
> 
> Best regards,

So if you're actually targetting EAS I don't get why you would check overutilized (instead
of asym_fits, what about uclamp?) but also, given that many EAS systems have only one common
llc I don't quite get why you would want this anyway.
Do you have a system / workload showing a benefit?
(I find with EAS heavily relying on wakeups, what we do in the slow path isn't that important
for most workloads...)