linux-kernel - Re: [PATCH 19/19] workqueue: implement concurrency managed workqueue

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Date:	Thu, 1 Oct 2009 10:55:31 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
cc:	jeff@...zik.org, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com, arjan@...ux.intel.com
Subject: Re: [PATCH 19/19] workqueue: implement concurrency managed
 workqueue

On Fri, 2 Oct 2009, Tejun Heo wrote:
>
> Yeap, this one is pretty scary.  I tried pretty hard to at least make
> the diffs fall into relevant sections (ie. updates to certain part of
> logic shows up as diffs to the original part) but I agree this patch
> is a tad too big.  Maybe I can introduce gcwq first.  The big problem
> is the strong inter-dependency between the single global worker pool
> and the hotplug logic.  I tried to introduce the hotplug logic first
> but it basically required implementing different interim mechanism
> altogether.  One solution could be disabling or crippling cpu hotplug
> support for several commits so that the processing part and hotplug
> part can be introduced separately.  Would that be acceptable?

I would say that disabling CPU hotplug for a while is likely a good way to 
at least limit the damage, except for one thing - suspend and resume. 

That's one of the things that is hard to debug anyway, is tied to 
workqueues (with that whole freezing thing) _and_ uses CPU hotplug to 
actually get its work done. So if you effectively disable CPU hotplug for 
a part of the series, then that just means that bisection doesn't work 
over that part - and is probably one of the areas where it would be the 
most important that it works ;^/

So I dunno. I do like the series even as-is. It seems to have fairly solid 
reasoning behind it, and I absolutely hate the deadlocks we have now in 
workqueue handling and the insane things we do to work around them. At the 
same time, this code is something I get worried about - I can easily 
imaging subtle bugs that only happen under certain circumstances and with 
certain timings - and the last patch was the scariest of the lot.

So if the thing could be split up while limiting CPU hotplug only a bit 
(so that the case of suspend/resume would hopefully still be ok), that 
would be wonderful.

			Linus
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