Re: [rfc] lru_add_drain_all() vs isolation

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Mon, 2009-09-07 at 16:18 +0200, Oleg Nesterov wrote:

> > flush_workqueue() could limit itself to cpus that had work queued since
> > the last flush_workqueue() invocation, etc.
> 
> But "work queued since the last flush_workqueue() invocation" just means
> "has work queued". Please note that flush_cpu_workqueue() does nothing
> if there are no works, except it does lock/unlock of cwq->lock.
> 
> IIRC, flush_cpu_workqueue() has to lock/unlock to avoid the races with
> CPU hotplug, but _perhaps_ flush_workqueue() can do the check lockless.
> 
> Afaics, we can add the workqueue_struct->cpu_map_has_works to help
> flush_workqueue(), but this means we should complicate insert_work()
> and run_workqueue() which should set/clear the bit. But given that
> flush_workqueue() should be avoided anyway, I am not sure.

Ah, indeed. Then nothing new would be needed here, since it will indeed
not interrupt processing on the remote cpus that never queued any work.