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Subject: Re: workqueue_set_max_active(wq, 0)?
From: Johannes Berg <johannes@sipsolutions.net>
To: Tejun Heo <tj@kernel.org>
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On Thu, 2011-12-15 at 11:12 -0800, Tejun Heo wrote:

> > Hm, good point. We can't abstract out all of it -- the freezer API
> > doesn't want to wait for it to finish -- but probably a bit of it.
> > 
> > How do you iterate workqueues? We'd have to do that for the freezer
> > part, unless we want to work on CWQs again.
> 
> By Locking workqueue_lock and walking workqueues list.  Hmmm...

Ah. So fundamentally, the freeze code does:

 * set each gcwq frozen
 * set max_active=0 for each CWQ in each WQ

but it interleaves the two loops. I guess this would have to be
untangled if we want to share it so it sets all gcwq frozen and then
iterates the workqueues and their CWQs. Locking seems a bit hairy
though, why does the current code keep the GCWQ lock over CWQ changes? I
guess that's so nothing can work on the CWQ?

> > Actually I'm not really sure I understand the differences between WQ,
> > CWQ and GCWQ...
> 
[snip explanation]

thanks.

> The reason why FREEZING currently is on GCWQ is because freezing is a
> system wide operation.  If we're gonna implement pause, I think it
> should probably be in cwq.

Ok, makes sense too.

I think I'm going to do something simpler first though, the locking
scares me a bit. I'll do something for my single-threaded max-active=1
workqueue first directly in mac80211 to try out the idea ...

johannes