linux-kernel - Re: [PATCH 21/32] fs/aio: aio_wq isn't used in memory reclaim path

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Message-ID: <20110105112802.GA7669@mtj.dyndns.org>
Date:	Wed, 5 Jan 2011 03:28:02 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Benjamin LaHaise <bcrl@...ck.org>,
	linux-aio@...ck.org
Subject: Re: [PATCH 21/32] fs/aio: aio_wq isn't used in memory reclaim path

Hello,

On Tue, Jan 04, 2011 at 10:56:20AM -0500, Jeff Moyer wrote:
> > aio_wq isn't used during memory reclaim.  Convert to alloc_workqueue()
> > without WQ_MEM_RECLAIM.  It's possible to use system_wq but given that
> > the number of work items is determined from userland and the work item
> > may block, enforcing strict concurrency limit would be a good idea.
> 
> I would think that just given that it may block would be enough to keep
> it off of the system workqueue.

Oh, workqueue now can handle parallel execution.  Blocking on system
workqueue is no longer a problem.  One of the main reasons for this
whole series.

> > @@ -85,7 +85,7 @@ static int __init aio_setup(void)
> >  	kiocb_cachep = KMEM_CACHE(kiocb, SLAB_HWCACHE_ALIGN|SLAB_PANIC);
> >  	kioctx_cachep = KMEM_CACHE(kioctx,SLAB_HWCACHE_ALIGN|SLAB_PANIC);
> >  
> > -	aio_wq = create_workqueue("aio");
> > +	aio_wq = alloc_workqueue("aio", 0, 1);	/* used to limit concurrency */
> 
> OK, the only difference here is the removal of the WQ_MEM_RECLAIM flag,
> as you noted.

Yeap.  Do you agree that the concurrency limit is necessary?  If not,
we can just put everything onto system_wq.

> > @@ -569,7 +569,7 @@ static int __aio_put_req(struct kioctx *ctx, struct kiocb *req)
> >  		spin_lock(&fput_lock);
> >  		list_add(&req->ki_list, &fput_head);
> >  		spin_unlock(&fput_lock);
> > -		queue_work(aio_wq, &fput_work);
> > +		schedule_work(&fput_work);
> 
> I'm not sure where this change fits into the patch description.  Why did
> you do this?

Yeah, that's me being forgetful.  Now that aio_wq is solely used to
throttle the max concurrency of aio work items, I thought it would be
better to push fput_work to system workqueue so that it doesn't
interact with aio work items.  I'll update the patch description.

Thanks.

-- 
tejun
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