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: Andrea Righi <arighi@nvidia.com>
Acked-by: Changwoo Min <changwoo@igalia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index a175b622716ceea9eb006efcc505ef559b28779a..6e530a91e94424a7bf49680f9ba86be63eab9115 100644 (file)
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -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.