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From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>,
	axboe@kernel.dk, ctalbott@google.com, rni@google.com,
	linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
	containers@lists.linux-foundation.org
Subject: Re: [PATCH 07/11] blkcg: make request_queue bypassing on allocation
Date: Tue, 17 Apr 2012 16:04:58 +0400	[thread overview]
Message-ID: <1334664298.3766.62.camel@dabdike> (raw)
In-Reply-To: <20120413211640.GH12233@google.com>

On Fri, 2012-04-13 at 14:16 -0700, Tejun Heo wrote:
> On Fri, Apr 13, 2012 at 02:05:48PM -0700, Tejun Heo wrote:
> > On Fri, Apr 13, 2012 at 04:55:01PM -0400, Vivek Goyal wrote:
> > > But neither seems to be the case here. So to make sure that blkg_lookup()
> > > under rcu will see the updated value of queue flag (bypass), are we
> > > relying on the fact that caller should see the DEAD flag and not go
> > > ahead with blkg_lookup()?  If yes, atleast it is not obivious.
> > 
> > We're relying on the fact that it doesn't matter anymore because all
> > blkgs will be shoot down in queue cleanup path which goes through rcu
> > free, which is different from deactivating individual policies.  It
> > indeed is subtle.  Umm... this is starting to get ridiculous.  Why the
> > hell was megaraid messing with so many queues anyways?
> 
> I suppose megaraid depends on sequential LUN scan which SCSI
> implements by creating sdev for each LUN, trying to see whether it
> actually exists and then destroys the sdev if not.  Urgh.... so, we
> seem to be stuck with it.

Right, sorry ... it's not just megaraid, it's any SCSI-2 device.  The
standard says we have to probe the LUNs one at a time to see if they're
there.  SCSI-3 on supports the REPORT LUNS command which just returns a
list which obviates the need to probe on every one but not all older
(and USB to be frank) devices support this.

> So, the current code is technically correct although subtle like hell.
> We can RCU defer blk_put_queue() from blk_cleanup_queue() using
> call_rcu() to make clear that RCU grace period is necessary there.
> Any better ideas?

Not really ... except that perhaps we might redo LUN scanning to use
just a single queue, so repurpose the LUN underneath, but not destroy
the old queue and setup the new one?  It's a bit counter intuitive, but
it shouldn't be impossible.

James



  parent reply	other threads:[~2012-04-17 12:05 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-04-13 20:11 [PATCHSET] block: per-queue policy activation, take#2 Tejun Heo
2012-04-13 20:11 ` [PATCH 01/11] cfq: fix build breakage & warnings Tejun Heo
2012-04-13 20:11 ` [PATCH 02/11] blkcg: kill blkio_list and replace blkio_list_lock with a mutex Tejun Heo
2012-04-13 20:11 ` [PATCH 03/11] blkcg: use @pol instead of @plid in update_root_blkg_pd() and blkcg_print_blkgs() Tejun Heo
2012-04-13 20:11 ` [PATCH 04/11] blkcg: remove static policy ID enums Tejun Heo
2012-04-13 20:11 ` [PATCH 05/11] blkcg: make blkg_conf_prep() take @pol and return with queue lock held Tejun Heo
2012-04-13 20:11 ` [PATCH 06/11] blkcg: make sure blkg_lookup() returns %NULL if @q is bypassing Tejun Heo
2012-04-13 21:50   ` [PATCH UPDATED " Tejun Heo
2012-04-13 20:11 ` [PATCH 07/11] blkcg: make request_queue bypassing on allocation Tejun Heo
2012-04-13 20:32   ` Vivek Goyal
2012-04-13 20:37     ` Tejun Heo
2012-04-13 20:44       ` Vivek Goyal
2012-04-13 20:47         ` Tejun Heo
2012-04-13 20:55           ` Vivek Goyal
2012-04-13 21:05             ` Tejun Heo
2012-04-13 21:16               ` Tejun Heo
2012-04-13 21:31                 ` Tejun Heo
2012-04-17 12:04                 ` James Bottomley [this message]
2012-04-18 21:42                   ` Tejun Heo
2012-04-13 21:33               ` Vivek Goyal
2012-04-13 21:38                 ` Tejun Heo
2012-04-16 12:39                   ` Vivek Goyal
2012-04-13 20:37     ` Vivek Goyal
2012-04-13 20:11 ` [PATCH 08/11] blkcg: add request_queue->root_blkg Tejun Heo
2012-04-13 20:11 ` [PATCH 09/11] blkcg: implement per-queue policy activation Tejun Heo
2012-04-13 20:11 ` [PATCH 10/11] blkcg: drop stuff unused after per-queue policy activation update Tejun Heo
2012-04-13 20:11 ` [PATCH 11/11] blkcg: shoot down blkgs if all policies are deactivated Tejun Heo
2012-04-20  8:09 ` [PATCHSET] block: per-queue policy activation, take#2 Jens Axboe
2012-04-20 12:02   ` Jens Axboe
2012-04-20 17:17     ` Tejun Heo
2012-04-20 19:08       ` Jens Axboe
2012-04-25 18:19         ` Tejun Heo

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