On 1/19/18 4:52 PM, Ming Lei wrote: > On Fri, Jan 19, 2018 at 10:38:41AM -0700, Jens Axboe wrote: >> 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 > > If null_blk's timer mode is used with a bit delay introduced, I guess > the effect from direct access to underlying queue shouldn't be so > serious. But it still won't be good as direct access. Doesn't matter if it's used at the default of 10usec completion latency, or inline complete. Same result, I ran both. > Another way may be to introduce a variants blk_get_request(), such as > blk_get_request_with_notify(), then pass the current dm-rq's hctx to > it, and use the tag's waitqueue to handle that. But the change can be > a bit big. Yes, that's exactly the solution I suggested both yesterday and today. -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
