linux-kernel - Re: Overview of concurrency managed workqueue

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Message-ID: <1276695734.3640.20.camel@jlt3.sipsolutions.net>
Date:	Wed, 16 Jun 2010 15:42:14 +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 Wed, 2010-06-16 at 15:39 +0200, Tejun Heo wrote:
> On 06/16/2010 03:37 PM, Johannes Berg wrote:
> >> 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.
> 
> Oh yeah, if you have WQ_SINGLE_CPU + max inflight of 1, works on the
> wq are fully ordered.

Ok, great, thanks. FWIW, that's pretty much all I care about right
now :)

johannes

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