From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752930AbdKTUwT (ORCPT ); Mon, 20 Nov 2017 15:52:19 -0500 Received: from mail-it0-f44.google.com ([209.85.214.44]:41236 "EHLO mail-it0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752896AbdKTUwQ (ORCPT ); Mon, 20 Nov 2017 15:52:16 -0500 X-Google-Smtp-Source: AGs4zMZpdI/k9Y/ANCWPIllkNGC+iO8Prf4fbYAEVfa+dHktT27Kh+Ly/ryKIzKNSR9BBPoAiJoXkg== Subject: Re: 4.14: WARNING: CPU: 4 PID: 2895 at block/blk-mq.c:1144 with virtio-blk To: Christian Borntraeger , Bart Van Assche , "virtualization@lists.linux-foundation.org" , "linux-block@vger.kernel.org" , "mst@redhat.com" , "jasowang@redhat.com" , "linux-kernel@vger.kernel.org" References: <9c5eec5d-f542-4d76-6933-6fe31203ce09@de.ibm.com> <1511205644.2396.32.camel@wdc.com> <04526c98-ffc5-1eca-3aa8-50f9212c4323@de.ibm.com> <5c9f2228-0a8b-8225-7038-e6cb3f31ca0b@kernel.dk> <2e44dbd3-2f90-c267-560c-91d1d4b0e892@de.ibm.com> From: Jens Axboe Message-ID: <823b9dd5-7781-5a72-03ff-bc931433fc19@kernel.dk> Date: Mon, 20 Nov 2017 13:52:14 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 MIME-Version: 1.0 In-Reply-To: <2e44dbd3-2f90-c267-560c-91d1d4b0e892@de.ibm.com> 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 11/20/2017 01:49 PM, Christian Borntraeger wrote: > > > On 11/20/2017 08:42 PM, Jens Axboe wrote: >> On 11/20/2017 12:29 PM, Christian Borntraeger wrote: >>> >>> >>> On 11/20/2017 08:20 PM, Bart Van Assche wrote: >>>> On Fri, 2017-11-17 at 15:42 +0100, Christian Borntraeger wrote: >>>>> This is >>>>> >>>>> b7a71e66d (Jens Axboe 2017-08-01 09:28:24 -0600 1141) * are mapped to it. >>>>> b7a71e66d (Jens Axboe 2017-08-01 09:28:24 -0600 1142) */ >>>>> 6a83e74d2 (Bart Van Assche 2016-11-02 10:09:51 -0600 1143) WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) && >>>>> 6a83e74d2 (Bart Van Assche 2016-11-02 10:09:51 -0600 1144) cpu_online(hctx->next_cpu)); >>>>> 6a83e74d2 (Bart Van Assche 2016-11-02 10:09:51 -0600 1145) >>>>> b7a71e66d (Jens Axboe 2017-08-01 09:28:24 -0600 1146) /* >>>> >>>> Did you really try to figure out when the code that reported the warning >>>> was introduced? I think that warning was introduced through the following >>>> commit: >>> >>> This was more a cut'n'paste to show which warning triggered since line numbers are somewhat volatile. >>> >>>> >>>> commit fd1270d5df6a005e1248e87042159a799cc4b2c9 >>>> Date: Wed Apr 16 09:23:48 2014 -0600 >>>> >>>> blk-mq: don't use preempt_count() to check for right CPU >>>> >>>> UP or CONFIG_PREEMPT_NONE will return 0, and what we really >>>> want to check is whether or not we are on the right CPU. >>>> So don't make PREEMPT part of this, just test the CPU in >>>> the mask directly. >>>> >>>> Anyway, I think that warning is appropriate and useful. So the next step >>>> is to figure out what work item was involved and why that work item got >>>> executed on the wrong CPU. >>> >>> It seems to be related to virtio-blk (is triggered by fio on such disks). Your comment basically >>> says: "no this is not a known issue" then :-) >>> I will try to take a dump to find out the work item >> >> blk-mq does not attempt to freeze/sync existing work if a CPU goes away, >> and we reconfigure the mappings. So I don't think the above is unexpected, >> if you are doing CPU hot unplug while running a fio job. > > I did a cpu hot plug (adding a CPU) and I started fio AFTER that. OK, that's different, we should not be triggering a warning for that. What does your machine/virtblk topology look like in terms of CPUS, nr of queues for virtblk, etc? You can probably get this info the easiest by just doing a: # find /sys/kernel/debug/block/virtX replace virtX with your virtblk device name. Generate this info both before and after the hotplug event. >> While it's a bit annoying that we trigger the WARN_ON() for a condition >> that can happen, we're basically interested in it if it triggers for >> normal operations. > > I think we should never trigger a WARN_ON on conditions that can > happen. I know some folks enabling panic_on_warn to detect/avoid data > integrity issues. FWIW, this also seems to happen wit 4.13 and 4.12 It's not supposed to happen for your case, so I'd say it's been useful. It's not a critical thing, but it is something that should not trigger and we need to look into why it did, and fixing it up. -- Jens Axboe