Subject: Re: Overview of concurrency managed workqueue
From: Johannes Berg <johannes@sipsolutions.net>
To: Tejun Heo <tj@kernel.org>
Cc: mingo@elte.hu, awalls@radix.net, linux-kernel@vger.kernel.org,
       jeff@garzik.org, akpm@linux-foundation.org, rusty@rustcorp.com.au,
       cl@linux-foundation.org, dhowells@redhat.com, arjan@linux.intel.com,
       oleg@redhat.com, axboe@kernel.dk
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References: <1276551467-21246-1-git-send-email-tj@kernel.org>
	 <4C17C598.7070303@kernel.org>
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Date: Wed, 16 Jun 2010 15:37:48 +0200
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On Tue, 2010-06-15 at 20:25 +0200, Tejun Heo wrote:

> cmwq extends workqueue with focus on the following goals.
> 
> * Workqueue is already very widely used.  Maintain compatibility with
>   the current API while removing limitations of the current
>   implementation.

...

> As multiple execution contexts are available for each wq, deadlocks
> around execution contexts is much harder to create.  The default
> workqueue, system_wq, has maximum concurrency level of 256 and unless
> there is a use case which can result in a dependency loop involving
> more than 254 workers, it won't deadlock.

I see a lot of stuff about the current limitations etc., but nothing
about code that actually _relies_ on the synchronisation properties of
the current wqs. We talked about that a long time ago, is it still
guaranteed that a single-threaded wq will serialise all work put onto
it? It needs to be, but I don't see you explicitly mentioning it.

johannes

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