Commit 9390a923 authored by Tejun Heo's avatar Tejun Heo
Browse files

sched_ext: Improve comment on idle_sched_class exception in scx_task_iter_next_locked() 



scx_task_iter_next_locked() skips tasks whose sched_class is
idle_sched_class. While it has a short comment explaining why it's testing
the sched_class directly isntead of using is_idle_task(), the comment
doesn't sufficiently explain what's going on and why. Improve the comment.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: default avatarDavid Vernet <void@manifault.com>
parent a735d43c

kernel/sched/ext.c

+23 −2
Original line number Diff line number Diff line
@@ -1252,8 +1252,29 @@ scx_task_iter_next_locked(struct scx_task_iter *iter, bool include_dead) 


	while ((p = scx_task_iter_next(iter))) {

		/*

		 * is_idle_task() tests %PF_IDLE which may not be set for CPUs

		 * which haven't yet been onlined. Test sched_class directly.

		 * scx_task_iter is used to prepare and move tasks into SCX

		 * while loading the BPF scheduler and vice-versa while

		 * unloading. The init_tasks ("swappers") should be excluded

		 * from the iteration because:

		 *

		 * - It's unsafe to use __setschduler_prio() on an init_task to

		 *   determine the sched_class to use as it won't preserve its

		 *   idle_sched_class.

		 *

		 * - ops.init/exit_task() can easily be confused if called with

		 *   init_tasks as they, e.g., share PID 0.

		 *

		 * As init_tasks are never scheduled through SCX, they can be

		 * skipped safely. Note that is_idle_task() which tests %PF_IDLE

		 * doesn't work here:

		 *

		 * - %PF_IDLE may not be set for an init_task whose CPU hasn't

		 *   yet been onlined.

		 *

		 * - %PF_IDLE can be set on tasks that are not init_tasks. See

		 *   play_idle_precise() used by CONFIG_IDLE_INJECT.

		 *

		 * Test for idle_sched_class as only init_tasks are on it.

		 */

		if (p->sched_class != &idle_sched_class)

			break;