linux-kernel - Re: [PATCH] sched/fair: Prefer cache-hot prev

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAKfTPtAVzgq5HxVc6q9VnD8i9wFtAWH0tnLHD81TBAudj2yFmA@mail.gmail.com>
Date: Wed, 22 Oct 2025 18:54:21 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: shubhang@...amperecomputing.com
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, Shubhang Kaushik <sh@...two.org>, 
	Shijie Huang <Shijie.Huang@...erecomputing.com>, Frank Wang <zwang@...erecomputing.com>, 
	Christopher Lameter <cl@...two.org>, Adam Li <adam.li@...erecomputing.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Prefer cache-hot prev_cpu for wakeup

On Sat, 18 Oct 2025 at 01:01, Shubhang Kaushik via B4 Relay
<devnull+shubhang.os.amperecomputing.com@...nel.org> wrote:
>
> From: Shubhang Kaushik <shubhang@...amperecomputing.com>
>
> Modify the wakeup path in `select_task_rq_fair()` to prioritize cache
> locality for waking tasks. The previous fast path always attempted to
> find an idle sibling, even if the task's prev CPU was not truly busy.
>
> The original problem was that under some circumstances, this could lead
> to unnecessary task migrations away from a cache-hot core, even when
> the task's prev CPU was a suitable candidate. The scheduler's internal
> mechanism `cpu_overutilized()` provide an evaluation of CPU load.
>
> To address this, the wakeup heuristic is updated to check the status of
> the task's `prev_cpu` first:
> - If the `prev_cpu` is  not overutilized (as determined by
>   `cpu_overutilized()`, via PELT), the task is woken up on
>   its previous CPU. This leverages cache locality and avoids
>   a potentially unnecessary migration.
> - If the `prev_cpu` is considered busy or overutilized, the scheduler
>   falls back to the existing behavior of searching for an idle sibling.
>
> Signed-off-by: Shubhang Kaushik <shubhang@...amperecomputing.com>
> ---
> This patch optimizes the scheduler's wakeup path to prioritize cache
> locality by keeping a task on its previous CPU if it is not overutilized,
> falling back to a sibling search only when necessary.
> ---
>  kernel/sched/fair.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bc0b7ce8a65d6bbe616953f530f7a02bb619537c..bb0d28d7d9872642cb5a4076caeb3ac9d8fe7bcd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8618,7 +8618,16 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>                 new_cpu = sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag);
>         } else if (wake_flags & WF_TTWU) { /* XXX always ? */
>                 /* Fast path */
> -               new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
> +
> +               /*
> +                * Avoid wakeup on an overutilized CPU.
> +                * If the previous CPU is not overloaded, retain the same for cache locality.
> +                * Otherwise, search for an idle sibling.
> +                */
> +               if (!cpu_overutilized(prev_cpu))

cpu_overutilized() returns false if (!sched_energy_enabled())

so  new_cpu is always prev_cpu for non EAS aware system which is
probably not what you want

> +                       new_cpu = prev_cpu;
> +               else
> +                       new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
>         }
>         rcu_read_unlock();
>
>
> ---
> base-commit: 9b332cece987ee1790b2ed4c989e28162fa47860
> change-id: 20251017-b4-sched-cfs-refactor-propagate-2c4a820998a4
>
> Best regards,
> --
> Shubhang Kaushik <shubhang@...amperecomputing.com>
>
>