Re: [PATCHSET] workqueue: reimplement CPU hotplug to keep idle workers

[Tejun Heo <tj@kernel.org>]

Hello, Peter.

On Fri, Jul 20, 2012 at 07:01:40PM +0200, Peter Zijlstra wrote:
> On Fri, 2012-07-20 at 09:52 -0700, Tejun Heo wrote:
> > Ooh, yeah, I agree.  That's next on the wq to-do list.  The problem is
> > that queue_work() is implemented in terms of queue_work_on().  
> 
> But that's trivial to fix, both could use __queue_work() without too
> much bother, right?

Hmmm?  Not really.  We need to keep count of the ones which reqested
fixed binding - ie. the ones which explicitly used queue_work_on() -
and then flush on wq CPU_DOWN.  Then, we need to audit the current
users which are using queue_work{_on}() + explicit FLUSH on CPU_DOWN
and convert them.

> > The assumption was that they should flush during CPU_DOWN but it
> > probably will be much better to require users which need CPU affinity
> > to always use queue_work_on() - instead of implicit local affinity
> > from queue_work() - and flush them automatically from wq callback.
> 
> Right, and when you create this new mode, which you need to know to
> flush on DOWN, you can simply put a BUG_ON in queue_work_on() when this
> mode is set.

BUG_ON() on queue_work_on()?  Do you mean if the target CPU is down?
If so, yeah, I'd probably go with WARN_ON_ONCE() but we should whine
on it.

Thanks.

-- 
tejun