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sched_ext: Clarify CPU context for running/stopping callbacks

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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: Andrea Righi <arighi@nvidia.com>
Acked-by: Changwoo Min <changwoo@igalia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Andrea Righi
2025-04-23 23:02:05 +02:00
committed by Tejun Heo
parent ac47c272b2
commit 069ac9e161
1 changed files with 18 additions and 0 deletions
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18
kernel/sched/ext.c
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@@ -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.