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

[Shubhang Kaushik OS <Shubhang@os.amperecomputing.com>]

> From your previous answer on v1, I don't think that you use
> heterogeneous system so eas will not be enabled in your case and even
> when used find_energy_efficient_cpu() will be called before

I agree that the EAS centric approach in the current patch is misplaced for our homogeneous systems.

> Otherwise you might want to check in wake_affine() where we decide
> between local cpu and previous cpu which one should be the target.
> This can have an impact especially if there are not in the same LLC

While wake_affine() modifications seem logical, I see that they cause performance regressions across the board due to the inherent trade-offs in altering that critical initial decision point.
We might need to solve the non-idle fallback within `select_idle_sibling` to ring fence the impact for preserving locality effectively.

Thanks,
Shubhang Kaushik

________________________________________
From: Vincent Guittot <vincent.guittot@linaro.org>
Sent: Monday, November 3, 2025 1:04 AM
To: Shubhang Kaushik OS
Cc: Ingo Molnar; Peter Zijlstra; Juri Lelli; Dietmar Eggemann; Steven Rostedt; Ben Segall; Mel Gorman; Valentin Schneider; Shubhang Kaushik; Shijie Huang; Frank Wang; Christopher Lameter; Adam Li; linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] sched/fair: Prefer cache locality for EAS wakeup

On Thu, 30 Oct 2025 at 20:19, Shubhang Kaushik via B4 Relay
<devnull+shubhang.os.amperecomputing.com@kernel.org> 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))

From your previous answer on v1, I don't think that you use
heterogeneous system so eas will not be enabled in your case and even
when used find_energy_efficient_cpu() will be called before

select_idle_sibling looks for an idle cpu that shares the cache with
target, Isn't such migration inside the same LLC good in your case ?

Otherwise you might want to check in wake_affine() where we decide
between local cpu and previous cpu which one should be the target.
This can have an impact especially if there are not in the same LLC

> +               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,
> --
> Shubhang Kaushik <shubhang@os.amperecomputing.com>
>
>