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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
