[PATCH] sched/core: Remove redundant core_sched_seq

[Hui Su]

core_sched_seq records whether an rq has consumed the pick made by the
last core-wide selection.

However, rq->core_pick already carries the same per-rq state. A core-wide
selection leaves core_pick populated only for siblings which still need to
consume their picks. It is cleared when the current CPU consumes its pick,
when a sibling is already running the selected task, or when a pending pick
is consumed through the fastpath. The CPU offline path clears it as well.

core_task_seq and core_pick_seq serve a separate purpose. core_task_seq
changes when the task set changes and for each new core-wide pick, while
core_pick_seq records the task sequence on which the selection was made.
Their equality therefore establishes that a pending core_pick is still
valid.

Consequently, once core_pick_seq == core_task_seq, a non-NULL core_pick is
sufficient to tell that this rq still has a pick to consume.
core_sched_seq duplicates that pending/consumed state.

Remove core_sched_seq and use core_pick directly as the pending marker.

Testing included an instrumented comparison of the old and new pending
predicates under core-scheduling stress. Additional guest tests exercised
repeated core-cookie lifecycles, forced idle, and CPU hotplug under load.
No predicate divergence or kernel warning was observed.

Signed-off-by: Hui Su <sh_def@xxxxxxx>
---
 kernel/sched/core.c  | 10 ++++------
 kernel/sched/sched.h |  1 -
 2 files changed, 4 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7885ff76e69f..6025d37532c6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6276,10 +6276,7 @@ pick_next_task(struct rq *rq, struct rq_flags *rf)
 	 * selection. In this case, do a core-wide selection.
 	 */
 	if (rq->core->core_pick_seq == rq->core->core_task_seq &&
-	    rq->core->core_pick_seq != rq->core_sched_seq &&
 	    rq->core_pick) {
-		WRITE_ONCE(rq->core_sched_seq, rq->core->core_pick_seq);
-
 		next = rq->core_pick;
 		rq->dl_server = rq->core_dl_server;
 		rq->core_pick = NULL;
@@ -6311,11 +6308,13 @@ pick_next_task(struct rq *rq, struct rq_flags *rf)
 	}
 
 	/*
-	 * core->core_task_seq, core->core_pick_seq, rq->core_sched_seq
+	 * core->core_task_seq, core->core_pick_seq
 	 *
 	 * @task_seq guards the task state ({en,de}queues)
 	 * @pick_seq is the @task_seq we did a selection on
-	 * @sched_seq is the @pick_seq we scheduled
+	 *
+	 * Once a core-wide selection is committed, a non-NULL core_pick denotes
+	 * a pick which still needs to be consumed on this CPU.
 	 *
 	 * However, preemptions can cause multiple picks on the same task set.
 	 * 'Fix' this by also increasing @task_seq for every pick.
@@ -6422,7 +6421,6 @@ pick_next_task(struct rq *rq, struct rq_flags *rf)
 
 	rq->core->core_pick_seq = rq->core->core_task_seq;
 	next = rq->core_pick;
-	rq->core_sched_seq = rq->core->core_pick_seq;
 
 	/* Something should have been selected for current CPU */
 	WARN_ON_ONCE(!next);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e656c7059bf8..d1441bf6b4ef 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1371,7 +1371,6 @@ struct rq {
 	struct task_struct	*core_pick;
 	struct sched_dl_entity	*core_dl_server;
 	unsigned int		core_enabled;
-	unsigned int		core_sched_seq;
 	struct rb_root		core_tree;
 
 	/* shared state -- careful with sched_core_cpu_deactivate() */

base-commit: 587858367581b9c55c3690f4e63382ad622719d4
-- 
2.55.0