sched_ext: keep running prev when prev->scx.slice != 0

When %SCX_OPS_ENQ_LAST is set and prev->scx.slice != 0,
@Prev will be dispacthed into the local DSQ in put_prev_task_scx().
However, pick_task_scx() is executed before put_prev_task_scx(),
so it will not pick @Prev.
Set %SCX_RQ_BAL_KEEP in balance_one() to ensure that pick_task_scx()
can pick @Prev.

Signed-off-by: Henry Huang <henry.hj@antgroup.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: Henry Huang

kernel/sched/ext.c

@@ -2747,6 +2747,7 @@ static int balance_one(struct rq *rq, struct task_struct *prev)
 {
 	struct scx_dsp_ctx *dspc = this_cpu_ptr(scx_dsp_ctx);
 	bool prev_on_scx = prev->sched_class == &ext_sched_class;
+	bool prev_on_rq = prev->scx.flags & SCX_TASK_QUEUED;
 	int nr_loops = SCX_DSP_MAX_LOOPS;
 
 	lockdep_assert_rq_held(rq);
@@ -2779,8 +2780,7 @@ static int balance_one(struct rq *rq, struct task_struct *prev)
 		 * See scx_ops_disable_workfn() for the explanation on the
 		 * bypassing test.
 		 */
-		if ((prev->scx.flags & SCX_TASK_QUEUED) &&
-		    prev->scx.slice && !scx_rq_bypassing(rq)) {
+		if (prev_on_rq && prev->scx.slice && !scx_rq_bypassing(rq)) {
 			rq->scx.flags |= SCX_RQ_BAL_KEEP;
 			goto has_tasks;
 		}
@@ -2813,6 +2813,10 @@ static int balance_one(struct rq *rq, struct task_struct *prev)
 
 		flush_dispatch_buf(rq);
 
+		if (prev_on_rq && prev->scx.slice) {
+			rq->scx.flags |= SCX_RQ_BAL_KEEP;
+			goto has_tasks;
+		}
 		if (rq->scx.local_dsq.nr)
 			goto has_tasks;
 		if (consume_global_dsq(rq))
@@ -2838,8 +2842,7 @@ static int balance_one(struct rq *rq, struct task_struct *prev)
 	 * Didn't find another task to run. Keep running @prev unless
 	 * %SCX_OPS_ENQ_LAST is in effect.
 	 */
-	if ((prev->scx.flags & SCX_TASK_QUEUED) &&
-	    (!static_branch_unlikely(&scx_ops_enq_last) ||
+	if (prev_on_rq && (!static_branch_unlikely(&scx_ops_enq_last) ||
 	     scx_rq_bypassing(rq))) {
 		rq->scx.flags |= SCX_RQ_BAL_KEEP;
 		goto has_tasks;