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
