On 1/19/18 9:37 AM, Ming Lei wrote: > 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. Here's an example of that, using my current block tree (merged into master). The setup is dm-mpath on top of null_blk, the latter having just a single request. Both are mq devices. Fio direct 4k random reads on dm_mq: ~250K iops Start dd on underlying device (or partition on same device), just doing sequential reads. Fio direct 4k random reads on dm_mq with dd running: 9 iops No schedulers involved. https://i.imgur.com/WTDnnwE.gif -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
