[dm-devel] [RFC PATCH v2] dm mpath: add a queue_if_no_path timeout

[Frank Mayhar]

On Wed, 2013-10-30 at 11:43 -0400, Mike Snitzer wrote:
> On Wed, Oct 30 2013 at 11:08am -0400,
> Frank Mayhar <fmayhar at google.com> wrote:
> 
> > On Tue, 2013-10-29 at 21:02 -0400, Mike Snitzer wrote:
> > > Any interest in this or should I just table it for >= v3.14?
> > 
> > Sorry, I've been busy putting out another fire.  Yes, there's definitely
> > still interest.  I grabbed your revised patch and tested with it.
> > Unfortunately the timeout doesn't actually fire when requests are queued
> > due to queue_if_no_path; IIRC the block request queue timeout logic
> > wasn't triggering.  I planned to look into it more deeply figure out why
> > but I had to spend all last week fixing a nasty race and hadn't gotten
> > back to it yet.
> 
> OK, Hannes, any idea why this might be happening?  The patch in question
> is here: https://patchwork.kernel.org/patch/3070391/

I got to this today and so far the most interesting I see is that the
cloned request that's queued in multipath has no queue associated with
it when it's queued; a printk reveals:

[  517.610042] map_io: queueing rq ffff8801150e0070 q           (null)

When it's eventually dequeued, it gets a queue from the destination
device (in the pgpath) via bdev_get_queue().

Because of this and from just looking at the code, blk_start_request()
(and therefore blk_add_timer()) isn't being called for those requests,
so there's never a chance that the timeout would happen.

Does this make sense?  Or am I totally off-base?
-- 
Frank Mayhar
310-460-4042