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
