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