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    SubjectRe: [dm-devel] Re: [PATCH 1/1] scsi: export busy state via q->lld_busy_fn()
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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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