Subject: Re: [PATCH 17/32] scsi/ibmvstgt: use system_wq instead of vtgtd
 workqueue
From: James Bottomley <James.Bottomley@suse.de>
To: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: tj@kernel.org, bvanassche@acm.org, linux-kernel@vger.kernel.org,
        linux-scsi@vger.kernel.org, brking@linux.vnet.ibm.com,
        rcj@linux.vnet.ibm.com
In-Reply-To: <20110201231836D.fujita.tomonori@lab.ntt.co.jp>
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Date: Tue, 01 Feb 2011 08:25:32 -0600
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On Tue, 2011-02-01 at 23:18 +0900, FUJITA Tomonori wrote:
> On Tue, 1 Feb 2011 11:40:43 +0100
> Tejun Heo <tj@kernel.org> wrote:
> 
> > On Mon, Jan 24, 2011 at 05:24:14PM +0100, Tejun Heo wrote:
> > > Hello,
> > > 
> > > On Mon, Jan 24, 2011 at 05:09:18PM +0100, Bart Van Assche wrote:
> > > > Insertion of  flush_work_sync() fixes a race - that's a good catch.
> > > > flush_work_sync() should be invoked a little earlier though because
> > > > the scheduled work may access the queue destroyed by the
> > > > crq_queue_destroy(target) call. And the CRQ interrupt should be
> > > > disabled from before flush_work_sync() is invoked until after the CRQ
> > > > has been destroyed.
> > > 
> > > Heh, I'm a bit out of my depth here.  If you know what's necessary,
> > > please go ahead and make the change.
> > > 
> > > > Regarding the queue removal: I might have missed something, but why
> > > > would you like to remove the vtgtd work queue ? Since the ibmvstgt
> > > > driver is a storage target driver, processing latency matters. I'm
> > > > afraid that switching from a dedicated queue to the global work queue
> > > > will increase processing latency.
> > > 
> > > Having a dedicated workqueue no longer makes any difference regarding
> > > processing latency.  Each workqueue is mere frontend to the shared
> > > worker pool anyway.  Dedicated workqueues are now meaningful only as
> > > forward progress guarantee, attribute and/or flush domain - IOW, when
> > > the workqueue needs to be used during memory reclaim, the work items
> > > need to have specific attributes or certain group of work items need
> > > to be flushed together.  Apart from that, there's virtually no
> > > difference between using the system_wq and a dedicated one.  As using
> > > the system one is usually simpler, it's natural to do that.
> > 
> > Ping.  Are you interested in doing the conversion?
> 
> FYI, this driver will be replaced shortly. Now I have the working
> ibmvscsis driver for the new target framework. I'll submit it this
> week. So this driver will be removed soon or later (if James prefer to
> go through the proper Documentation/feature-removal-schedule.txt
> process, it'll be for some time). You could leave this alone, I guess.

Whatever works for you is fine by me.  I don't think we need to go
through feature removal since we're not technically removing the
feature.

James


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