Date: Sat, 04 Oct 2008 14:14:12 -0400 (EDT)
Message-Id: <20081004.141412.71086616.k-ueda@ct.jp.nec.com>
To: James.Bottomley@HansenPartnership.com
Cc: dm-devel@redhat.com, michaelc@cs.wisc.edu, linux-scsi@vger.kernel.org,
       linux-kernel@vger.kernel.org, j-nomura@ce.jp.nec.com,
       akpm@linux-foundation.org, k-ueda@ct.jp.nec.com
Subject: Re: [dm-devel] Re: [PATCH 1/1] scsi: export busy state via
 q->lld_busy_fn()
From: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
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	<20081003.160623.48532879.k-ueda@ct.jp.nec.com>
	<1223070706.3230.54.camel@localhost.localdomain>
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Hi James,

On Fri, 03 Oct 2008 16:51:46 -0500, James Bottomley wrote:
> On Fri, 2008-10-03 at 16:06 -0400, Kiyoshi Ueda wrote:
> > Hi James,
> > 
> > On Fri, 03 Oct 2008 13:14:56 -0500, James Bottomley wrote:
> > > On Wed, 2008-10-01 at 14:50 -0400, Kiyoshi Ueda wrote:
> > > > Hi James,
> > > > 
> > > > I hope the previous RFC patch(*) would be no problem, since I haven't
> > > > gotten any negative comment.
> > > >     (*) http://lkml.org/lkml/2008/9/25/262
> > > > 
> > > > So could you take this patch for 2.6.28 additionally?
> > > > This patch implements a new interface of the block layer for
> > > > request stacking drivers.
> > > > There should be no effect on existing drivers' behavior.
> > > > 
> > > > This patch was created on top of the commit below of scsi-post-merge-2.6.
> > > > ---------------------------------------------------------------------
> > > > commit e49f03f37104c0e7cae7c455480069bada00932f
> > > > Author: James Bottomley <James.Bottomley@HansenPartnership.com>
> > > > Date:   Fri Sep 12 16:46:51 2008 -0500
> > > > 
> > > >     [SCSI] scsi_error: fix target reset handling
> > > > ---------------------------------------------------------------------
> > > > 
> > > > And this patch depends on the following block layer patch, which
> > > > is in Jens' for-2.6.28 of linux-2.6-block.
> > > >     http://lkml.org/lkml/2008/9/29/142
> > > > 
> > > > Thanks,
> > > > Kiyoshi Ueda
> > > > 
> > > > 
> > > > Subject: [PATCH 1/1] scsi: export busy state via q->lld_busy_fn()
> > > > 
> > > > This patch implements q->lld_busy_fn() for scsi mid layer to export
> > > > its busy state.
> > > > 
> > > > Please note that it checks the cached information (sdev->lld_busy)
> > > > for efficiency, instead of checking the actual state of
> > > > sdev/starget/shost everytime.
> > > > 
> > > > So the care must be taken not to leave sdev->lld_busy = 1 for
> > > > the following cases:
> > > >     - when there is no request in the sdev queue
> > > >     - when scsi can't dispatch I/Os anymore and needs to kill I/Os
> > > > Otherwise, request stacking drivers may not submit any request,
> > > > and then, nobody sets back lld_busy = 0 and that causes deadlock.
> > > > 
> > > > OTOH, it has no major problem in setting sdev->lld_busy = 0 even when
> > > > the starget/shost is actually busy, because newly submitted request
> > > > will soon turn it to 1 in scsi_request_fn().
> > > > 
> > > > 
> > > > Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
> > > > Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
> > > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > > Cc: Mike Christie <michaelc@cs.wisc.edu>
> > > > Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
> > > > ---
> > > >  drivers/scsi/scsi.c        |    4 ++--
> > > >  drivers/scsi/scsi_lib.c    |   20 +++++++++++++++++++-
> > > >  include/scsi/scsi_device.h |   13 +++++++++++++
> > > >  3 files changed, 34 insertions(+), 3 deletions(-)
> > > > 
> > > > Index: scsi-post-merge-2.6/drivers/scsi/scsi_lib.c
> > > > ===================================================================
> > > > --- scsi-post-merge-2.6.orig/drivers/scsi/scsi_lib.c
> > > > +++ scsi-post-merge-2.6/drivers/scsi/scsi_lib.c
> > > > @@ -480,6 +480,8 @@ void scsi_device_unbusy(struct scsi_devi
> > > >  	spin_unlock(shost->host_lock);
> > > >  	spin_lock(sdev->request_queue->queue_lock);
> > > >  	sdev->device_busy--;
> > > > +	if (sdev->device_busy < sdev->queue_depth && !sdev->device_blocked)
> > > > +		sdev->lld_busy = 0;
> > > >  	spin_unlock_irqrestore(sdev->request_queue->queue_lock, flags);
> > > >  }
> > > >  
> > > > @@ -1535,6 +1537,13 @@ static void scsi_softirq_done(struct req
> > > >  	}
> > > >  }
> > > >  
> > > > +static int scsi_lld_busy(struct request_queue *q)
> > > > +{
> > > > +	struct scsi_device *sdev = q->queuedata;
> > > > +
> > > > +	return sdev ? sdev->lld_busy : 0;
> > > > +}
> > > 
> > > Since you've moved to a functional approach, why is this lld_busy flag
> > > still necessary?  Surely this function can just check the three blocked
> > > conditions and the two overqueue ones at this point without ever having
> > > to reach into any of the SCSI internals?
> > 
> > This flag is not necessary for the functionality, it's for efficiency.
> > I could take the "everytime checking" approach above, but I think
> > caching the busy state into the flag is more efficient, since:
> > 
> >   - The check function will be called from request stacking drivers
> >     frequently (e.g. request-based dm calls it everytime before
> >     an request is dispatched from the dm device.)
> 
> Hmm, well, so will the operations you've put into the stack, but for
> every request, not just for stacked drivers.
> 
> >   - The scsi busy state checking needs queue lock and host lock
> >     even while the scsi busy state doesn't changed from the previous
> >     checking.
> 
> You don't need the locks ... you don't care if the values change as you
> read them because this is just heuristic.  You do care that you get an
> architectural guarantee that you read a consistent value (as in if the
> variable is altering you read either the before or after) but this isn't
> necessary as the algorithm is statistical: as long as it gets bogus
> values infrequently the effects won't be noticeable different.

Thank you for the good suggestion.
I sent another patch-set: http://lkml.org/lkml/2008/10/4/85

Thanks,
Kiyoshi Ueda
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