Commit 069ac9e1 authored by Andrea Righi's avatar Andrea Righi Committed by Tejun Heo
Browse files

sched_ext: Clarify CPU context for running/stopping callbacks 

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.

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>
Signed-off-by: default avatarAndrea Righi <arighi@nvidia.com>
Acked-by: default avatarChangwoo Min <changwoo@igalia.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent ac47c272

kernel/sched/ext.c

+18 −0
Original line number Diff line number Diff line
@@ -373,6 +373,15 @@ 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);
@@ -382,6 +391,15 @@ 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.