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SubjectRe: Workqueues splat due to ending up on wrong CPU
On Mon, Dec 02, 2019 at 12:13:38PM -0800, Tejun Heo wrote:
> Hello, Paul.
>
> (cc'ing scheduler folks - workqueue rescuer is very occassionally
> triggering a warning which says that it isn't on the cpu it should be
> on under rcu cpu hotplug torture test. It's checking smp_processor_id
> is the expected one after a successful set_cpus_allowed_ptr() call.)
>
> On Sun, Dec 01, 2019 at 05:55:48PM -0800, Paul E. McKenney wrote:
> > > And hyperthreading seems to have done the trick! One splat thus far,
> > > shown below. The run should complete this evening, Pacific Time.
> >
> > That was the only one for that run, but another 24*56-hour run got three
> > more. All of them expected to be on CPU 0 (which never goes offline, so
> > why?) and the "XXX" diagnostic never did print.
>
> Heh, I didn't expect that, so maybe set_cpus_allowed_ptr() is
> returning 0 while not migrating the rescuer task to the target cpu for
> some reason?
>
> The rescuer is always calling to migrate itself, so it must always be
> running. set_cpus_allowed_ptr() migrates live ones by calling
> stop_one_cpu() which schedules a migration function which runs from a
> highpri task on the target cpu. Please take a look at the following.
>
> static bool cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
> {
> ...
> enabled = stopper->enabled;
> if (enabled)
> __cpu_stop_queue_work(stopper, work, &wakeq);
> else if (work->done)
> cpu_stop_signal_done(work->done);
> ...
> }
>
> So, if stopper->enabled is clear, it'll signal completion without
> running the work.

Is there ever a valid case for this? That is, why isn't that a WARN()?

> stopper->enabled is cleared during cpu hotunplug
> and restored from bringup_cpu() while cpu is being brought back up.
>
> static int bringup_wait_for_ap(unsigned int cpu)
> {
> ...
> stop_machine_unpark(cpu);
> ....
> }
>
> static int bringup_cpu(unsigned int cpu)
> {
> ...
> ret = __cpu_up(cpu, idle);
> ...
> return bringup_wait_for_ap(cpu);
> }
>
> __cpu_up() is what marks the cpu online and once the cpu is online,
> kthreads are free to migrate into the cpu, so it looks like there's a
> brief window where a cpu is marked online but the stopper thread is
> still disabled meaning that a kthread may schedule into the cpu but
> not out of it, which would explain the symptom that you were seeing.

Yes.

> It could be that I'm misreading the code. What do you guys think?

The below seems to not insta explode...

---
kernel/cpu.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index a59cc980adad..9eaedd002f41 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -525,8 +525,7 @@ static int bringup_wait_for_ap(unsigned int cpu)
if (WARN_ON_ONCE((!cpu_online(cpu))))
return -ECANCELED;

- /* Unpark the stopper thread and the hotplug thread of the target cpu */
- stop_machine_unpark(cpu);
+ /* Unpark the hotplug thread of the target cpu */
kthread_unpark(st->thread);

/*
@@ -1089,8 +1088,8 @@ void notify_cpu_starting(unsigned int cpu)

/*
* Called from the idle task. Wake up the controlling task which brings the
- * stopper and the hotplug thread of the upcoming CPU up and then delegates
- * the rest of the online bringup to the hotplug thread.
+ * hotplug thread of the upcoming CPU up and then delegates the rest of the
+ * online bringup to the hotplug thread.
*/
void cpuhp_online_idle(enum cpuhp_state state)
{
@@ -1100,6 +1099,12 @@ void cpuhp_online_idle(enum cpuhp_state state)
if (state != CPUHP_AP_ONLINE_IDLE)
return;

+ /*
+ * Unpark the stopper thread before we start the idle thread; this
+ * ensures the stopper is always available.
+ */
+ stop_machine_unpark(smp_processor_id());
+
st->state = CPUHP_AP_ONLINE_IDLE;
complete_ap_thread(st, true);
}
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