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

[Waiman Long <longman@redhat.com>]

On 6/27/24 08:16, Hillf Danton 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.
>>
> Wonder if the MCS lock does not help in this case.

This patch just tries to avoid polluting the shared cacheline leading to 
excessive cacheline bouncing. No locking is involved. I am not sure what 
you are thinking about using MCS lock for.

Regards,
Longman

>>    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
>>