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. > 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
