Re: [PATCH] workqueue: fix rebind bound workers warning

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, 11 May 2016, Wanpeng Li wrote:
> Hi Tejun,
> 2016-05-10 5:50 GMT+08:00 Wanpeng Li <kernellwp@gmail.com>:
> > Cc Thomas, the new state machine author,
> > 2016-05-10 1:00 GMT+08:00 Tejun Heo <tj@kernel.org>:
> >> Hello,
> >>
> >> On Thu, May 05, 2016 at 09:41:31AM +0800, Wanpeng Li wrote:
> >>> The boot CPU handles housekeeping duty(unbound timers, workqueues,
> >>> timekeeping, ...) on behalf of full dynticks CPUs. It must remain
> >>> online when nohz full is enabled. There is a priority set to every
> >>> notifier_blocks:
> >>>
> >>> workqueue_cpu_up > tick_nohz_cpu_down > workqueue_cpu_down
> >>>
> >>> So tick_nohz_cpu_down callback failed when down prepare cpu 0, and
> >>> notifier_blocks behind tick_nohz_cpu_down will not be called any
> >>> more, which leads to workers are actually not unbound. Then hotplug
> >>> state machine will fallback to undo and online cpu 0 again. Workers
> >>> will be rebound unconditionally even if they are not unbound and
> >>> trigger the warning in this progress.
> >>
> >> I'm a bit confused.  Are you saying that the hotplug statemachine may
> >> invoke CPU_DOWN_FAILED w/o preceding CPU_DOWN on the same callback?
> >
> > I think so. CPU_DOWN_FAILED is detected in the process of CPU_DOWN_PREPARE

Well, no. It's not detected.

If a down prepare callback fails, then DOWN_FAILED is invoked for all
callbacks which have successfully executed DOWN_PREPARE.

But, workqueue has actually two notifiers. One which handles
UP/DOWN_FAILED/ONLINE and one which handles DOWN_PREPARE.

Now look at the priorities of those callbacks:

    CPU_PRI_WORKQUEUE_UP	= 5
    CPU_PRI_WORKQUEUE_DOWN	= -5

So the call order on DOWN_PREPARE is:

   CB 1
   CB ...
   CB workqueue_up() -> Ignores DOWN_PREPARE
   CB ...
   CB X ---> Fails

So we call up to CB X with DOWN_FAILED

   CB 1
   CB ...
   CB workqueue_up() -> Handles DOWN_FAILED
   CB ...
   CB X-1

So the problem is that the workqueue stuff handles DOWN_FAILED in the up
callback, while it should do it in the down callback. Which is not a good idea
either because it wants to be called early on rollback...

Brilliant stuff, isn't it? The hotplug rework will solve this problem because
the callbacks become symetric, but for the existing mess, we need some
workaround in the workqueue code.

Thanks,

	tglx