linux-kernel - Re: [PATCH 15/19] workqueue: reimplement workqueue flushing using color coded works

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Message-ID: <4AC4E405.4040407@kernel.org>
Date:	Fri, 02 Oct 2009 02:16:53 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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 15/19] workqueue: reimplement workqueue flushing using
 color coded works

Tejun Heo wrote:
> Hello, Linus.
> 
> Linus Torvalds wrote:
>> On Thu, 1 Oct 2009, Tejun Heo wrote:
>>> Reimplement workqueue flushing using color coded works.  There are two
>>> colors and each cwq has the current color which is painted on the
>>> works being issued via the cwq.  Flushing a workqueue is achieved by
>>> flipping the current colors of each cwq and wait for the works which
>>> have the old color to drain.
>> Is there any reason for the "two colors" choice? I could imagine that it 
>> could end up being a limitation (and possible deadlock?) to allow just a 
>> single flush pending at any time.
>>
>> Could the color be an 8-bit counter or something like that instead?
> 
> It's related to how many bits can be used from work_struct->data which
> in turn is determined by the alignment of cwq.  Currently, the
> alignment is 8 bytes so 3 bits are available.  One is used for
> PENDING, the other for LINKED and one last bit is used for COLOR.
> Aligning cwq to, say, 64bytes wouldn't be bad at all and then we can
> have 6-bits of coloring.  Hmmm... yeap, I'll work on that.

Oops, 4 not 6.

-- 
tejun
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