linux-kernel - Re: workqueue_set_max

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Date:	Thu, 15 Dec 2011 11:12:12 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: workqueue_set_max_active(wq, 0)?

Hello,

On Thu, Dec 15, 2011 at 07:43:40PM +0100, Johannes Berg 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...

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

WQ is workqueue - the part visible to users.

CWQ is cpu workqueue.  Each wq has its own set of cpu workqueues for
all CPUs (there are exceptions but this should be a good enough
explanation).  A WQ is always a set of cwq's.  WQ chooses which CWQ to
use on queue but most of actual processing happens on CWQs.

GCWQ stands for global cpu workqueue - there's one for each CPU.  This
is per-cpu global worker pool used by all workqueues.  Every CWQ on a
CPU shares the GCWQ on that CPU.

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.

> > I think it would be great if this can be abstracted out so that both
> > the freezer and explicit pausing use the same facility.  They aren't
> > that different after all.
> 
> I'll take a look tomorrow. If you want to beat me to it ... ;-)

Heh heh, [un]fortunately, I'm pretty occupied at the moment.  :P

Thank you.

-- 
tejun
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