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

[Ming Lei]

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.

-- 
Ming