From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932447AbeASRiw (ORCPT ); Fri, 19 Jan 2018 12:38:52 -0500 Received: from mail-io0-f175.google.com ([209.85.223.175]:35586 "EHLO mail-io0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932231AbeASRio (ORCPT ); Fri, 19 Jan 2018 12:38:44 -0500 X-Google-Smtp-Source: ACJfBou9WxOK7CByWxky2vCec9bNQrf0cSry7vSGwFR6fdoSsFQbp1+rEkeRCwu8PySXTzrPTH/4sg== Subject: Re: [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle To: Ming Lei Cc: Bart Van Assche , "snitzer@redhat.com" , "dm-devel@redhat.com" , "hch@infradead.org" , "linux-kernel@vger.kernel.org" , "linux-block@vger.kernel.org" , "osandov@fb.com" References: <20180119023212.GA25413@ming.t460p> <20180119072623.GB25369@ming.t460p> <047f68ec-f51b-190f-2f89-f413325c2540@kernel.dk> <20180119154047.GB14827@ming.t460p> <540e1239-c415-766b-d4ff-bb0b7f3517a7@kernel.dk> <20180119160518.GC14827@ming.t460p> <4a5c049f-0fab-bbaf-bfe2-eb5bca73f2c8@kernel.dk> <20180119162618.GD14827@ming.t460p> <1f072086-533e-4b75-d0e3-9e621b2120d8@kernel.dk> <20180119163736.GE14827@ming.t460p> From: Jens Axboe Message-ID: Date: Fri, 19 Jan 2018 10:38:41 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:58.0) Gecko/20100101 Thunderbird/58.0 MIME-Version: 1.0 In-Reply-To: <20180119163736.GE14827@ming.t460p> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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