sched_ext: Track the cpu a task is runnable on

Add p->scx.runnable_cpu, the cpu @p is runnable on, or -1 when it is not.
It is stamped as @p joins the runnable_list (set_task_runnable()) and
cleared as it leaves (clr_task_runnable()), both under the rq lock.

task_cpu() can't answer "is @p on this rq" reliably: a remote wakeup changes
it under @p's pi_lock alone, without the source rq lock, so it can read as
the locked rq while @p is really elsewhere. runnable_cpu changes only under
the rq lock, so a caller holding an rq lock can compare against it to know
whether that is @p's current rq.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 7e3f6b33f4a8..853f03b63133 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -201,6 +201,7 @@ struct sched_ext_entity {
 	s32			sticky_cpu;
 	s32			holding_cpu;
 	s32			selected_cpu;
+	s32			runnable_cpu;	/* cpu @p is runnable on, -1 if not */
 	struct task_struct	*kf_tasks[2];	/* see SCX_CALL_OP_TASK() */
 
 	struct list_head	runnable_node;	/* rq->scx.runnable_list */