Re: Workqueues splat due to ending up on wrong CPU

["Paul E. McKenney" <paulmck@kernel.org>]

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.  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.
> 
> This makes the cpumask and the cpu the task is actually on disagree
> and retries would become noops.  I can work around it by excluding
> rescuer attachments against hotplugs but this looks like a genuine cpu
> hotplug bug.
> 
> It could be that I'm misreading the code.  What do you guys think?

I think that I do not understand the code, but I never let that stop
me from asking stupid questions!  ;-)

Suppose that a given worker is bound to a particular CPU, but has no
work pending, and is therefore sleeping in the schedule() call near the
end of worker_thread().  During this time, its CPU goes offline and then
comes back online.  Doesn't this break that task's affinity to that CPU?
Then the call to workqueue_online_cpu() is supposed to rebind all the
tasks that might have been affected, correct?

I could imagine putting a trace_printk() or two in workqueue_online_cpu()
and adding the task_struct pointer (or PID) to the WARN_ONCE(), though I
am worried that this might decrease the race probability.

Is there a better way to proceed?

							Thanx, Paul