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

[Ming Lei]

On Fri, Jan 19, 2018 at 09:23:35AM -0700, Jens Axboe wrote:
> On 1/19/18 9:13 AM, Mike Snitzer wrote:
> > On Fri, Jan 19 2018 at 10:48am -0500,
> > Jens Axboe <axboe@xxxxxxxxx> 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.
> > 
> > Right, #2 is _not_ the way forward.  Historically request-based DM used
> > its own mempool for requests, this was to be able to have some measure
> > of control and resiliency in the face of low memory conditions that
> > might be affecting the broader system.
> > 
> > Then Christoph switched over to adding per-request-data; which ushered
> > in the use of blk_get_request using ATOMIC allocations.  I like the
> > result of that line of development.  But taking the next step of setting
> > BLK_MQ_F_BLOCKING is highly unfortunate (especially in that this
> > dm-mpath.c code is common to old .request_fn and blk-mq, at least the
> > call to blk_get_request is).  Ultimately dm-mpath like to avoid blocking
> > for a request because for this dm-mpath device we have multiple queues
> > to allocate from if need be (provided we have an active-active storage
> > network topology).
> 
> If you can go to multiple devices, obviously it should not block on a
> single device. That's only true for the case where you can only go to
> one device, blocking at that point would probably be fine. Or if all
> your paths are busy, then blocking would also be OK.

Introducing one extra block point will hurt AIO performance, in which
there is usually much less jobs/processes to submit IO.

-- 
Ming