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sched_ext: TASK_DEAD tasks must be switched out of SCX on ops_disable

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scx_ops_disable_workfn() only switches !TASK_DEAD tasks out of SCX while
calling scx_ops_exit_task() on all tasks including dead ones. This can leave
a dead task on SCX but with SCX_TASK_NONE state, which is inconsistent.

If another task was in the process of changing the TASK_DEAD task's
scheduling class and grabs the rq lock after scx_ops_disable_workfn() is
done with the task, the task ends up calling scx_ops_disable_task() on the
dead task which is in an inconsistent state triggering a warning:

  WARNING: CPU: 6 PID: 3316 at kernel/sched/ext.c:3411 scx_ops_disable_task+0x12c/0x160
  ...
  RIP: 0010:scx_ops_disable_task+0x12c/0x160
  ...
  Call Trace:
   <TASK>
   check_class_changed+0x2c/0x70
   __sched_setscheduler+0x8a0/0xa50
   do_sched_setscheduler+0x104/0x1c0
   __x64_sys_sched_setscheduler+0x18/0x30
   do_syscall_64+0x7b/0x140
   entry_SYSCALL_64_after_hwframe+0x76/0x7e
  RIP: 0033:0x7f140d70ea5b

There is no reason to leave dead tasks on SCX when unloading the BPF
scheduler. Fix by making scx_ops_disable_workfn() eject all tasks including
the dead ones from SCX.

Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tejun Heo 2024-08-30 13:44:40 -10:00
parent 37cb049ef8
commit 61eeb9a905
1 changed files with 8 additions and 16 deletions
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14
kernel/sched/ext.c
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@ -3998,21 +3998,14 @@ static void scx_ops_disable_workfn(struct kthread_work *work)
spin_lock_irq(&scx_tasks_lock);
scx_task_iter_init(&sti);
/*
* Invoke scx_ops_exit_task() on all non-idle tasks, including
* TASK_DEAD tasks. Because dead tasks may have a nonzero refcount,
* we may not have invoked sched_ext_free() on them by the time a
* scheduler is disabled. We must therefore exit the task here, or we'd
* fail to invoke ops.exit_task(), as the scheduler will have been
* unloaded by the time the task is subsequently exited on the
* sched_ext_free() path.
* The BPF scheduler is going away. All tasks including %TASK_DEAD ones
* must be switched out and exited synchronously.
*/
while ((p = scx_task_iter_next_locked(&sti, true))) {
const struct sched_class *old_class = p->sched_class;
struct sched_enq_and_set_ctx ctx;
if (READ_ONCE(p->__state) != TASK_DEAD) {
sched_deq_and_put_task(p, DEQUEUE_SAVE | DEQUEUE_MOVE,
&ctx);
sched_deq_and_put_task(p, DEQUEUE_SAVE | DEQUEUE_MOVE, &ctx);
p->scx.slice = min_t(u64, p->scx.slice, SCX_SLICE_DFL);
__setscheduler_prio(p, p->prio);
@ -4021,7 +4014,6 @@ static void scx_ops_disable_workfn(struct kthread_work *work)
sched_enq_and_set_task(&ctx);
check_class_changed(task_rq(p), p, old_class, p->prio);
}
scx_ops_exit_task(p);
}
scx_task_iter_exit(&sti);