linux-kernel - Re: [PATCH v2] USB: appledisplay: Remove deprecated create_singlethread

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Message-ID: <20160726165134.GV19588@mtj.duckdns.org>
Date:	Tue, 26 Jul 2016 12:51:34 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Bhaktipriya Shridhar <bhaktipriya96@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] USB: appledisplay: Remove deprecated
 create_singlethread_workqueue

On Tue, Jul 26, 2016 at 10:19:18PM +0530, Bhaktipriya Shridhar wrote:
> The workqueue "wq" is involved in controlling the brightness of an
> Apple Cinema Display over USB.
> 
> It has a single work item(&pdata->work) per appledisplay and hence
> doesn't require ordering. Also, it is not being used on a memory
> reclaim path.
> 
> Hence, the singlethreaded workqueue has been replaced with the use of
> system_wq.
> 
> System workqueues have been able to handle high level of concurrency
> for a long time now and hence it's not required to have a singlethreaded
> workqueue just to gain concurrency. Unlike a dedicated per-cpu workqueue
> created with create_singlethread_workqueue(), system_wq allows multiple
> work items to overlap executions even on the same CPU; however, a
> per-cpu workqueue doesn't have any CPU locality or global ordering
> guarantee unless the target CPU is explicitly specified and thus the
> increase of local concurrency shouldn't make any difference.
> 
> The work item is self-requeueing and needs to wait for the in-flight
> work item to finish before proceeding with destruction.
> Hence, it has been sync cancelled in appledisplay_disconnect().
> This also ensures that there are no pending tasks while disconnecting
> the driver.
> 
> Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@...il.com>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun