sched_ext: bypass mode shouldn't depend on ops.select_cpu()

Bypass mode was depending on ops.select_cpu() which can't be trusted as with
the rest of the BPF scheduler. Always enable and use scx_select_cpu_dfl() in
bypass mode.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: David Vernet <void@manifault.com>

kernel/sched/ext.c

@@ -3126,7 +3126,7 @@ static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flag
 	if (unlikely(wake_flags & WF_EXEC))
 		return prev_cpu;
 
-	if (SCX_HAS_OP(select_cpu)) {
+	if (SCX_HAS_OP(select_cpu) && !scx_rq_bypassing(task_rq(p))) {
 		s32 cpu;
 		struct task_struct **ddsp_taskp;
 
@@ -3191,7 +3191,7 @@ void __scx_update_idle(struct rq *rq, bool idle)
 {
 	int cpu = cpu_of(rq);
 
-	if (SCX_HAS_OP(update_idle)) {
+	if (SCX_HAS_OP(update_idle) && !scx_rq_bypassing(rq)) {
 		SCX_CALL_OP(SCX_KF_REST, update_idle, cpu_of(rq), idle);
 		if (!static_branch_unlikely(&scx_builtin_idle_enabled))
 			return;
@@ -4254,21 +4254,23 @@ bool task_should_scx(struct task_struct *p)
  * the DISABLING state and then cycling the queued tasks through dequeue/enqueue
  * to force global FIFO scheduling.
  *
- * a. ops.enqueue() is ignored and tasks are queued in simple global FIFO order.
+ * - ops.select_cpu() is ignored and the default select_cpu() is used.
+ *
+ * - ops.enqueue() is ignored and tasks are queued in simple global FIFO order.
  *   %SCX_OPS_ENQ_LAST is also ignored.
  *
- * b. ops.dispatch() is ignored.
+ * - ops.dispatch() is ignored.
  *
- * c. balance_scx() does not set %SCX_RQ_BAL_KEEP on non-zero slice as slice
+ * - 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.
  *
- * d. pick_next_task() suppresses zero slice warning.
+ * - pick_next_task() suppresses zero slice warning.
  *
- * e. scx_bpf_kick_cpu() is disabled to avoid irq_work malfunction during PM
+ * - scx_bpf_kick_cpu() is disabled to avoid irq_work malfunction during PM
  *   operations.
  *
- * f. scx_prio_less() reverts to the default core_sched_at order.
+ * - scx_prio_less() reverts to the default core_sched_at order.
  */
 static void scx_ops_bypass(bool bypass)
 {
@@ -4338,7 +4340,7 @@ static void scx_ops_bypass(bool bypass)
 
 		rq_unlock_irqrestore(rq, &rf);
 
-		/* kick to restore ticks */
+		/* resched to restore ticks and idle state */
 		resched_cpu(cpu);
 	}
 }