linux-kernel - Re: [PATCH 16/19] workqueue: reimplement workqueue flushing using color coded works

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Date:	Fri, 04 Dec 2009 12:46:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	torvalds@...ux-foundation.org, awalls@...ix.net,
	linux-kernel@...r.kernel.org, jeff@...zik.org, mingo@...e.hu,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
	johannes@...solutions.net
Subject: Re: [PATCH 16/19] workqueue: reimplement workqueue flushing using
 color coded works

On Fri, 2009-11-20 at 13:46 +0900, Tejun Heo wrote:
> Reimplement workqueue flushing using color coded works.  wq has the
> current work color which is painted on the works being issued via
> cwqs.  Flushing a workqueue is achieved by advancing the current work
> colors of cwqs and waiting for all the works which have any of the
> previous colors to drain.
> 
> Currently there are 16 colors allowing 15 concurrent flushes.  When
> color space gets full, flush attempts are batched up and processed
> together when color frees up, so even with many concurrent flushers,
> the new implementation won't build up huge queue of flushers which has
> to be processed one after another.
> 
> This new implementation leaves only cleanup_workqueue_thread() as the
> user of flush_cpu_workqueue().  Just make its users use
> flush_workqueue() and kthread_stop() directly and kill
> cleanup_workqueue_thread().  As workqueue flushing doesn't use barrier
> request anymore, the comment describing the complex synchronization
> around it in cleanup_workqueue_thread() is removed together with the
> function.
> 
> This new implementation is to allow having and sharing multiple
> workers per cpu.

Hmm, a long while back I did a PI aware workqueue implementation, that
used nested work-lists to implement barriers and flushing.

This colour thing seems very prone to starvation when you want to extend
worklets with priority.

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