[PATCH 5/6] sched_ext: Improve comment on idle_sched_class exception in scx_task_iter_next_locked()

Tejun Heo
2024-08-04T02:41:00Z

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: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
---
kernel/sched/ext.c | 25 +++++++++++++++++++++++--
1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 09f394bb4889..7837a551022c 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -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;
--
2.46.0