Re: BUG: workqueue lockup - SRCU schedules work on not-online CPUs during size transition

[Tejun Heo]

Hello, Paul.

On Fri, Apr 10, 2026 at 12:17:21PM -0700, Paul E. McKenney wrote:
> > The easiest way to do this is just creating the initial workers for all
> > possible pools. Please see below. However, the downside is that it's going
> > to create all workers for all possible cpus. This isn't a problem for
> > anybody else but these IBM mainframes often come up with a lot of possible
> > but not-yet-or-ever-online CPUs for capacity management, so the cost may not
> > be negligible on some configurations.
> > 
> > IBM folks, is that okay?
> 
> I have also seen x86 systems whose firmware claimed very large numbers
> of CPUs.  :-(

Yeah, I remember seeing those but at least the ones I remember are from long
times ago. Hopefully, no bios is getting things that wrong anymore.

> > Also, why do you need to queue work items on an offline CPU? Do they
> > actually have to be per-cpu? Can you get away with using an unbound
> > workqueue?
> 
> It is good for them to run on the specified CPU in the common case for
> cache-locality reasons, but if they were occasionally redirected to some
> other CPU, that would be just fine.

I see.  

> I am also keeping the patch that avoids queueing work to CPUs that are not
> yet fully online.  Further adjustments will be needed if someone invokes
> call_srcu(), synchronize_srcu(), or synchronize_srcu_expedited() from an
> CPU that is not yet fully online.  Past experience of course suggests that
> this will be happen, and that there will be a good reason for it.  ;-)

I'm gonna hold for now. From workqueue side, it's a really easy change, so
please let me know if this comes up again.

Thanks.

-- 
tejun