Re: [PATCH v2] sched_ext: Clarify CPU context for running/stopping callbacks

[Changwoo Min <changwoo@igalia.com>]

Hi Andrea,

On 4/24/25 06:02, 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 when a task
> property is changed, as both scx_next_task_scx() and dequeue_task_scx() may
> run on CPUs different from the task's target CPU.

The same goes to ops.quiescent() too since ops.quiescent() is also
called from dequeue_task_scx().

Reviewed-by: Changwoo Min <changwoo@igalia.com>

Regards,
Changwoo Min

> 
> 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 | 18 ++++++++++++++++++
>   1 file changed, 18 insertions(+)
> 
> Changes in v2:
>   - clarify the scenario a bit more in the code comments
>   - link to v1: https://lore.kernel.org/all/20250423190059.270236-1-arighi@nvidia.com/
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index ac79067dc87e6..a83232a032aa4 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -368,6 +368,15 @@ 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. This can happen when a task
> +	 * property is changed (i.e., affinity), since scx_next_task_scx(),
> +	 * which triggers this callback, may run on a CPU different from
> +	 * the task's assigned CPU.
> +	 *
> +	 * Therefore, always use scx_bpf_task_cpu(@p) to determine the
> +	 * target CPU the task is going to use.
> +	 *
>   	 * See ->runnable() for explanation on the task state notifiers.
>   	 */
>   	void (*running)(struct task_struct *p);
> @@ -377,6 +386,15 @@ struct sched_ext_ops {
>   	 * @p: task stopping to run
>   	 * @runnable: is task @p still runnable?
>   	 *
> +	 * Note that this callback may be called from a CPU other than the
> +	 * one the task was running on. This can happen when a task
> +	 * property is changed (i.e., affinity), since dequeue_task_scx(),
> +	 * which triggers this callback, may run on a CPU different from
> +	 * the task's assigned CPU.
> +	 *
> +	 * Therefore, always use scx_bpf_task_cpu(@p) to retrieve the CPU
> +	 * the task was running on.
> +	 *
>   	 * See ->runnable() for explanation on the task state notifiers. If
>   	 * !@runnable, ->quiescent() will be invoked after this operation
>   	 * returns.