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

[Hillf Danton]

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.
> 
Wonder if the MCS lock does not help in this case.

>   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
>