Re: [PATCH] sched_ext: Clarify CPU context for running/stopping callbacks

[Tejun Heo <tj@kernel.org>]

On Wed, Apr 23, 2025 at 09:00:59PM +0200, Andrea Righi wrote:
> The ops.running() and ops.stopping() callbacks can be invoked from a CPU
> other than the one the task is assigned to, particularly during affinity
> changes, as both scx_next_task_scx() and dequeue_task_scx() may run on
> CPUs different from the task's target CPU.
> 
> This behavior can lead to confusion or incorrect assumptions if not
> properly clarified, potentially resulting in bugs (see [1]).
> 
> Therefore, update the documentation to clarify this aspect and advise
> users to use scx_bpf_task_cpu() to determine the actual CPU the task
> will run on or was running on.
> 
> [1] https://github.com/sched-ext/scx/pull/1728
> 
> Cc: Jake Hillion <jake@hillion.co.uk>
> Cc: Changwoo Min <changwoo@igalia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/ext.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index a2380a6bba210..f146e678cc261 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -373,6 +373,10 @@ struct sched_ext_ops {
>  	 * @running: A task is starting to run on its associated CPU
>  	 * @p: task starting to run
>  	 *
> +	 * Note that this callback may be called from a CPU other than the
> +	 * one the task is going to run on. Use scx_bpf_task_cpu(@p) to
> +	 * determine the target CPU the task is going to use.

Can you briefly explain the scenario as a part of the comment? Just a couple
sentences can go a long way against future head scratches.

Thanks.

-- 
tejun