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sched_ext: Improve comment on idle_sched_class exception in scx_task_iter_next_locked()

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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>
This commit is contained in:
Tejun Heo
2024-08-06 09:40:11 -10:00
parent a735d43c7f
commit 9390a923e1
1 changed files with 23 additions and 2 deletions
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25
kernel/sched/ext.c
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@@ -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;