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authorTejun Heo <tj@kernel.org>2024-08-06 09:40:11 -1000
committerTejun Heo <tj@kernel.org>2024-08-06 09:40:11 -1000
commit9390a923e109f85b242bf676dc5bc81958d447fa (patch)
treeb889e0c4c8a09c040e5689936dca37386d8488c7 /kernel/sched/ext.c
parenta735d43c7f85d112a6aefd72973188d0626e4464 (diff)
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: Tejun Heo <tj@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Acked-by: David Vernet <void@manifault.com>
Diffstat (limited to 'kernel/sched/ext.c')
-rw-r--r--kernel/sched/ext.c25
1 files 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 @@ retry:
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;