From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932304AbeASQ16 (ORCPT ); Fri, 19 Jan 2018 11:27:58 -0500 Received: from mail-io0-f181.google.com ([209.85.223.181]:36641 "EHLO mail-io0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756102AbeASQ1t (ORCPT ); Fri, 19 Jan 2018 11:27:49 -0500 X-Google-Smtp-Source: ACJfBov7BYsSRjxiHRaWzsR8hldAckUzDVO2Oye/6konWPQn7wLnWzzxOqSIen1BTNQLslc9wgpgKw== 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: <1516301278.2676.35.camel@wdc.com> <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> From: Jens Axboe Message-ID: <1f072086-533e-4b75-d0e3-9e621b2120d8@kernel.dk> Date: Fri, 19 Jan 2018 09:27:46 -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: <20180119162618.GD14827@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 9:26 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. -- Jens Axboe