linux-kernel - Re: Overview of concurrency managed workqueue

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Message-ID: <1276695468.3640.19.camel@jlt3.sipsolutions.net>
Date:	Wed, 16 Jun 2010 15:37:48 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Tejun Heo <tj@...nel.org>
Cc:	mingo@...e.hu, awalls@...ix.net, linux-kernel@...r.kernel.org,
	jeff@...zik.org, akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com,
	arjan@...ux.intel.com, oleg@...hat.com, axboe@...nel.dk
Subject: Re: Overview of concurrency managed workqueue

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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