sched_ext: Replace SCX_TASK_BAL_KEEP with SCX_RQ_BAL_KEEP

SCX_TASK_BAL_KEEP is used by balance_one() to tell pick_next_task_scx() to
keep running the current task. It's not really a task property. Replace it
with SCX_RQ_BAL_KEEP which resides in rq->scx.flags and is a better fit for
the usage. Also, the existing clearing rule is unnecessarily strict and
makes it difficult to use with core-sched. Just clear it on entry to
balance_one().
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/sched/ext.h

@@ -71,7 +71,6 @@ struct scx_dispatch_q {
 /* scx_entity.flags */
 enum scx_ent_flags {
 	SCX_TASK_QUEUED		= 1 << 0, /* on ext runqueue */
-	SCX_TASK_BAL_KEEP	= 1 << 1, /* balance decided to keep current */
 	SCX_TASK_RESET_RUNNABLE_AT = 1 << 2, /* runnable_at should be reset */
 	SCX_TASK_DEQD_FOR_SLEEP	= 1 << 3, /* last dequeue was for SLEEP */
 

kernel/sched/ext.c

@@ -2508,6 +2508,7 @@ static int balance_one(struct rq *rq, struct task_struct *prev, bool local)
 
 	lockdep_assert_rq_held(rq);
 	rq->scx.flags |= SCX_RQ_IN_BALANCE;
+	rq->scx.flags &= ~SCX_RQ_BAL_KEEP;
 
 	if (static_branch_unlikely(&scx_ops_cpu_preempt) &&
 	    unlikely(rq->scx.cpu_released)) {
@@ -2523,7 +2524,6 @@ static int balance_one(struct rq *rq, struct task_struct *prev, bool local)
 	}
 
 	if (prev_on_scx) {
-		WARN_ON_ONCE(local && (prev->scx.flags & SCX_TASK_BAL_KEEP));
 		update_curr_scx(rq);
 
 		/*
@@ -2538,13 +2538,13 @@ static int balance_one(struct rq *rq, struct task_struct *prev, bool local)
 		 *
 		 * When balancing a remote CPU for core-sched, there won't be a
 		 * following put_prev_task_scx() call and we don't own
-		 * %SCX_TASK_BAL_KEEP. Instead, pick_task_scx() will test the
-		 * same conditions later and pick @rq->curr accordingly.
+		 * %SCX_RQ_BAL_KEEP. Instead, pick_task_scx() will test the same
+		 * conditions later and pick @rq->curr accordingly.
 		 */
 		if ((prev->scx.flags & SCX_TASK_QUEUED) &&
 		    prev->scx.slice && !scx_ops_bypassing()) {
 			if (local)
-				prev->scx.flags |= SCX_TASK_BAL_KEEP;
+				rq->scx.flags |= SCX_RQ_BAL_KEEP;
 			goto has_tasks;
 		}
 	}
@@ -2604,7 +2604,7 @@ static int balance_one(struct rq *rq, struct task_struct *prev, bool local)
 	if ((prev->scx.flags & SCX_TASK_QUEUED) &&
 	    (!static_branch_unlikely(&scx_ops_enq_last) || scx_ops_bypassing())) {
 		if (local)
-			prev->scx.flags |= SCX_TASK_BAL_KEEP;
+			rq->scx.flags |= SCX_RQ_BAL_KEEP;
 		goto has_tasks;
 	}
 	rq->scx.flags &= ~SCX_RQ_IN_BALANCE;
@@ -2726,8 +2726,6 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
 		SCX_CALL_OP_TASK(SCX_KF_REST, stopping, p, true);
 
 	if (p->scx.flags & SCX_TASK_QUEUED) {
-		p->scx.flags &= ~SCX_TASK_BAL_KEEP;
-
 		set_task_runnable(rq, p);
 
 		/*
@@ -2772,8 +2770,8 @@ static struct task_struct *pick_next_task_scx(struct rq *rq,
 	 * if necessary and keep running @prev. Otherwise, pop the first one
 	 * from the local DSQ.
 	 */
-	if (prev->scx.flags & SCX_TASK_BAL_KEEP) {
-		prev->scx.flags &= ~SCX_TASK_BAL_KEEP;
+	if ((rq->scx.flags & SCX_RQ_BAL_KEEP) &&
+	    !WARN_ON_ONCE(prev->sched_class != &ext_sched_class)) {
 		p = prev;
 		if (!p->scx.slice)
 			p->scx.slice = SCX_SLICE_DFL;
@@ -2841,7 +2839,7 @@ bool scx_prio_less(const struct task_struct *a, const struct task_struct *b,
  *
  * As put_prev_task_scx() hasn't been called on remote CPUs, we can't just look
  * at the first task in the local dsq. @rq->curr has to be considered explicitly
- * to mimic %SCX_TASK_BAL_KEEP.
+ * to mimic %SCX_RQ_BAL_KEEP.
  */
 static struct task_struct *pick_task_scx(struct rq *rq)
 {
@@ -3872,7 +3870,7 @@ bool task_should_scx(struct task_struct *p)
  *
  * b. ops.dispatch() is ignored.
  *
- * c. balance_scx() does not set %SCX_TASK_BAL_KEEP on non-zero slice as slice
+ * c. balance_scx() does not set %SCX_RQ_BAL_KEEP on non-zero slice as slice
  *    can't be trusted. Whenever a tick triggers, the running task is rotated to
  *    the tail of the queue with core_sched_at touched.
  *

kernel/sched/sched.h

@@ -737,6 +737,7 @@ enum scx_rq_flags {
 	 */
 	SCX_RQ_ONLINE		= 1 << 0,
 	SCX_RQ_CAN_STOP_TICK	= 1 << 1,
+	SCX_RQ_BAL_KEEP		= 1 << 2, /* balance decided to keep current */
 
 	SCX_RQ_IN_WAKEUP	= 1 << 16,
 	SCX_RQ_IN_BALANCE	= 1 << 17,