[dm-devel] [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle

[Ming Lei]

On Fri, Jan 19, 2018 at 09:27:46AM -0700, Jens Axboe wrote:
> On 1/19/18 9:26 AM, Ming Lei wrote:
> > On Fri, Jan 19, 2018 at 09:19:24AM -0700, Jens Axboe wrote:
> >> On 1/19/18 9:05 AM, Ming Lei wrote:
> >>> On Fri, Jan 19, 2018 at 08:48:55AM -0700, Jens Axboe wrote:
> >>>> On 1/19/18 8:40 AM, Ming Lei wrote:
> >>>>>>>> Where does the dm STS_RESOURCE error usually come from - what's exact
> >>>>>>>> resource are we running out of?
> >>>>>>>
> >>>>>>> It is from blk_get_request(underlying queue), see
> >>>>>>> multipath_clone_and_map().
> >>>>>>
> >>>>>> That's what I thought. So for a low queue depth underlying queue, it's
> >>>>>> quite possible that this situation can happen. Two potential solutions
> >>>>>> I see:
> >>>>>>
> >>>>>> 1) As described earlier in this thread, having a mechanism for being
> >>>>>>    notified when the scarce resource becomes available. It would not
> >>>>>>    be hard to tap into the existing sbitmap wait queue for that.
> >>>>>>
> >>>>>> 2) Have dm set BLK_MQ_F_BLOCKING and just sleep on the resource
> >>>>>>    allocation. I haven't read the dm code to know if this is a
> >>>>>>    possibility or not.
> >>>>>>
> >>>>>> I'd probably prefer #1. It's a classic case of trying to get the
> >>>>>> request, and if it fails, add ourselves to the sbitmap tag wait
> >>>>>> queue head, retry, and bail if that also fails. Connecting the
> >>>>>> scarce resource and the consumer is the only way to really fix
> >>>>>> this, without bogus arbitrary delays.
> >>>>>
> >>>>> Right, as I have replied to Bart, using mod_delayed_work_on() with
> >>>>> returning BLK_STS_NO_DEV_RESOURCE(or sort of name) for the scarce
> >>>>> resource should fix this issue.
> >>>>
> >>>> It'll fix the forever stall, but it won't really fix it, as we'll slow
> >>>> down the dm device by some random amount.
> >>>>
> >>>> A simple test case would be to have a null_blk device with a queue depth
> >>>> of one, and dm on top of that. Start a fio job that runs two jobs: one
> >>>> that does IO to the underlying device, and one that does IO to the dm
> >>>> device. If the job on the dm device runs substantially slower than the
> >>>> one to the underlying device, then the problem isn't really fixed.
> >>>
> >>> I remembered that I tried this test on scsi-debug & dm-mpath over scsi-debug,
> >>> seems not observed this issue, could you explain a bit why IO over dm-mpath
> >>> may be slower? Because both two IO contexts call same get_request(), and
> >>> in theory dm-mpath should be a bit quicker since it uses direct issue for
> >>> underlying queue, without io scheduler involved.
> >>
> >> Because if you lose the race for getting the request, you'll have some
> >> arbitrary delay before trying again, potentially. Compared to the direct
> > 
> > But the restart still works, one request is completed, then the queue
> > is return immediately because we use mod_delayed_work_on(0), so looks
> > no such issue.
> 
> There are no pending requests for this case, nothing to restart the
> queue. When you fail that blk_get_request(), you are idle, nothing
> is pending.

I think we needn't worry about that, once a device is attached to
dm-rq, it can't be mounted any more, and usually user don't use the device
directly and by dm-mpath at the same time.

-- 
Ming