sched, sched_ext: Open code for_balance_class_range()

For flexibility, sched_ext allows the BPF scheduler to select the CPU to
execute a task on at dispatch time so that e.g. a queue can be shared across
multiple CPUs. To enable this, the dispatch path is executed from balance()
so that a dispatched task can be hot-migrated to its target CPU. This means
that sched_ext needs its balance() method invoked before every
pick_next_task() even when the CPU is waking up from SCHED_IDLE.

for_balance_class_range() defined in kernel/sched/ext.h implements this
selective iteration promotion. However, the indirection obfuscates more than
helps. Open code the iteration promotion in put_prev_task_balance() and
remove for_balance_class_range().

No functional changes intended.

Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: David Vernet <void@manifault.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 13 insertions, 1 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7964edbe2ae6..a28e0daeae61 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5825,7 +5825,19 @@ static void put_prev_task_balance(struct rq *rq, struct task_struct *prev,
 				  struct rq_flags *rf)
 {
 #ifdef CONFIG_SMP
+	const struct sched_class *start_class = prev->sched_class;
 	const struct sched_class *class;
+
+#ifdef CONFIG_SCHED_CLASS_EXT
+	/*
+	 * SCX requires a balance() call before every pick_next_task() including
+	 * when waking up from SCHED_IDLE. If @start_class is below SCX, start
+	 * from SCX instead.
+	 */
+	if (sched_class_above(&ext_sched_class, start_class))
+		start_class = &ext_sched_class;
+#endif
+
 	/*
 	 * We must do the balancing pass before put_prev_task(), such
 	 * that when we release the rq->lock the task is in the same
@@ -5834,7 +5846,7 @@ static void put_prev_task_balance(struct rq *rq, struct task_struct *prev,
 	 * We can terminate the balance pass as soon as we know there is
 	 * a runnable task of @class priority or higher.
 	 */
-	for_balance_class_range(class, prev->sched_class, &idle_sched_class) {
+	for_active_class_range(class, start_class, &idle_sched_class) {
 		if (class->balance(rq, prev, rf))
 			break;
 	}