Re: [sched_ext/for-6.11]: Issue with BPF Scheduler during CPU Hotplug

[Tejun Heo <tj@kernel.org>]

On Tue, Aug 20, 2024 at 12:33:34PM +0530, Aboorva Devarajan wrote:
> On Tue, 2024-08-13 at 09:54 -1000, Tejun Heo wrote:
> > Hello,
> > 
> > On Sat, Aug 10, 2024 at 11:54:24PM +0530, Aboorva Devarajan wrote:
> > ...
> > ...
> 
> > Can you trigger sysrq-t when the system is stuck? Also, can you see whether
> > the problem is reproducible on x86 w/ the wq changes applied?
> 
> Hi Tejun,
> 
> I couldn't trigger sysrq-t so far as the system becomes unresponsive,
> will get back if I can successfully trigger this via console. 
> 
> And yes, this issue also occurs on x86 when applying the proposed workqueue
> patch on top of the recent sched-ext for-6.12 tree. However, it takes
> significantly longer to hit the issue, and the stack trace differs
> a bit.

I think this *should* fix the problem but it is rather ugly. The locking
order is such that there's no good way out. Maybe the solution is making
cpu_hotplug_disable() more useful. Anyways, can you test this one?

Thanks.

---
 kernel/sched/ext.c |   30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3990,6 +3990,24 @@ static const char *scx_exit_reason(enum
 	}
 }
 
+static void block_fork_hotplug(void)
+{
+	while (true) {
+		percpu_down_write(&scx_fork_rwsem);
+		if (cpus_read_trylock())
+			return;
+		percpu_up_write(&scx_fork_rwsem);
+		cpus_read_lock();
+		cpus_read_unlock();
+	}
+}
+
+static void unblock_fork_hotplug(void)
+{
+	cpus_read_unlock();
+	percpu_up_write(&scx_fork_rwsem);
+}
+
 static void scx_ops_disable_workfn(struct kthread_work *work)
 {
 	struct scx_exit_info *ei = scx_exit_info;
@@ -4045,8 +4063,7 @@ static void scx_ops_disable_workfn(struc
 	 * Avoid racing against fork. See scx_ops_enable() for explanation on
 	 * the locking order.
 	 */
-	percpu_down_write(&scx_fork_rwsem);
-	cpus_read_lock();
+	block_fork_hotplug();
 
 	spin_lock_irq(&scx_tasks_lock);
 	scx_task_iter_init(&sti);
@@ -4090,8 +4107,7 @@ static void scx_ops_disable_workfn(struc
 	static_branch_disable_cpuslocked(&scx_builtin_idle_enabled);
 	synchronize_rcu();
 
-	cpus_read_unlock();
-	percpu_up_write(&scx_fork_rwsem);
+	unblock_fork_hotplug();
 
 	if (ei->kind >= SCX_EXIT_ERROR) {
 		pr_err("sched_ext: BPF scheduler \"%s\" disabled (%s)\n",
@@ -4657,8 +4673,7 @@ static int scx_ops_enable(struct sched_e
 	 *
 	 *   scx_fork_rwsem --> pernet_ops_rwsem --> cpu_hotplug_lock
 	 */
-	percpu_down_write(&scx_fork_rwsem);
-	cpus_read_lock();
+	block_fork_hotplug();
 
 	check_hotplug_seq(ops);
 
@@ -4765,8 +4780,7 @@ static int scx_ops_enable(struct sched_e
 
 	spin_unlock_irq(&scx_tasks_lock);
 	preempt_enable();
-	cpus_read_unlock();
-	percpu_up_write(&scx_fork_rwsem);
+	unblock_fork_hotplug();
 
 	/* see above ENABLING transition for the explanation on exiting with 0 */
 	if (!scx_ops_tryset_enable_state(SCX_OPS_ENABLED, SCX_OPS_ENABLING)) {