Re: WARN_ON_ONCE() in process_one_work()?

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, May 01, 2017 at 02:44:02PM -0400, Tejun Heo wrote:
> Hello, Paul.
> 
> On Mon, May 01, 2017 at 11:38:07AM -0700, Paul E. McKenney wrote:
> > On Mon, May 01, 2017 at 09:57:47AM -0700, Paul E. McKenney wrote:
> > > Hello!
> > > 
> > > I am hitting this WARN_ON_ONCE() in process_one_work() and am wondering
> > > what I did wrong to make this happen:
> > 
> > Oh, wait...  Rescuer, it says.  Might this be due to the fact that RCU's
> > expedited grace periods block within a workqueue handler?  Might this
> > in turn run the system out of workqueue kthreads?  If this is the likely
> > cause, my approach would be to rework the expected-grace-period workqueue
> > handler to return when waiting for the grace period to complete, and to
> > replace the current wakeup with a schedule_work() or something similar.
> 
> That should be completely fine.  It could just be that the rescuer
> path has a bug around CPU hotplug handling.  Can you please confirm
> either way on the cpuset usage?

I have no explicit cpuset usage or affinity of the workqueue handlers
themselves.

However, this is thus far only happening in CONFIG_NO_HZ_FULL=y runs, in
this case, with the kernel boot parameter nohz_full=2-9 out of 16 CPUs.
IIRC, this sets up a "housekeeping" cpuset that pushes normal tasks away
from the nohz_full CPUs.

I do build with CONFIG_HOTPLUG_CPU=y, and the test does a lot of
hotplugging.  Also, other kthreads (but again, not the workqueue handlers)
do a lot of explicit CPU-affinity manipulation.

								Thanx, Paul