Re: [PATCH 2/4] workqueue: Improve scalability of workqueue watchdog touch

["Nicholas Piggin" <npiggin@gmail.com>]

On Wed Jun 26, 2024 at 2:57 AM AEST, Tejun Heo wrote:
> On Tue, Jun 25, 2024 at 09:42:45PM +1000, Nicholas Piggin wrote:
> > On a ~2000 CPU powerpc system, hard lockups have been observed in the
> > workqueue code when stop_machine runs (in this case due to CPU hotplug).
> > This is due to lots of CPUs spinning in multi_cpu_stop, calling
> > touch_nmi_watchdog() which ends up calling wq_watchdog_touch().
> > wq_watchdog_touch() writes to the global variable wq_watchdog_touched,
> > and that can find itself in the same cacheline as other important
> > workqueue data, which slows down operations to the point of lockups.
> > 
> > In the case of the following abridged trace, worker_pool_idr was in
> > the hot line, causing the lockups to always appear at idr_find.
> > 
> >   watchdog: CPU 1125 self-detected hard LOCKUP @ idr_find
> >   Call Trace:
> >   get_work_pool
> >   __queue_work
> >   call_timer_fn
> >   run_timer_softirq
> >   __do_softirq
> >   do_softirq_own_stack
> >   irq_exit
> >   timer_interrupt
> >   decrementer_common_virt
> >   * interrupt: 900 (timer) at multi_cpu_stop
> >   multi_cpu_stop
> >   cpu_stopper_thread
> >   smpboot_thread_fn
> >   kthread
> > 
> > Fix this by having wq_watchdog_touch() only write to the line if the
> > last time a touch was recorded exceeds 1/4 of the watchdog threshold.
> > 
> > Reported-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
>
> Applied 1-2 to wq/for-6.11.

Thanks Tejun.

> I think 3 and 4 should probably be routed
> through either tip or Andrew?

Yeah, let's see if it gets any comments.

Thanks,
Nick