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Message-ID: <ZtJZaKxg39SkQtMA@slm.duckdns.org>
Date: Fri, 30 Aug 2024 13:44:40 -1000
From: Tejun Heo <tj@...nel.org>
To: David Vernet <void@...ifault.com>
Cc: linux-kernel@...r.kernel.org, kernel-team@...a.com,
Andrea Righi <andrea.righi@...ux.dev>
Subject: [PATCH sched_ext/for-6.12] sched_ext: TASK_DEAD tasks must be
switched out of SCX on ops_disable
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@...nel.org>
---
kernel/sched/ext.c | 24 ++++++++----------------
1 file changed, 8 insertions(+), 16 deletions(-)
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 57f30b1604db..a1340d3c711c 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4051,30 +4051,22 @@ 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);
- check_class_changing(task_rq(p), p, old_class);
+ p->scx.slice = min_t(u64, p->scx.slice, SCX_SLICE_DFL);
+ __setscheduler_prio(p, p->prio);
+ check_class_changing(task_rq(p), p, old_class);
- sched_enq_and_set_task(&ctx);
+ sched_enq_and_set_task(&ctx);
- check_class_changed(task_rq(p), p, old_class, p->prio);
- }
+ check_class_changed(task_rq(p), p, old_class, p->prio);
scx_ops_exit_task(p);
}
scx_task_iter_exit(&sti);
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