On 6/12/18 10:19 AM, Chris Boot wrote: > On 12/06/18 17:09, Jens Axboe wrote: >> On 6/12/18 9:38 AM, Chris Boot wrote: >>> Hi folks, >>> >>> I maintain a large (to me) system with 112 threads (4x Intel E7-4830 v4) >>> which has a MegaRAID SAS 9361-24i controller. This system is currently >>> running Debian's 4.16.12 kernel (from stretch-backports) with blk_mq >>> enabled. >>> >>> I've run into a lockup which appears to involve blq_mq and writeback >>> throttling. It's hard to tell if I've run into this same thing with >>> older kernels; I'm trying to track down a deadlock but so far I've been >>> fairly certain that involved the OOM killer, but this doesn't seem to. > [snip] >> >> Hmm that's really weird, I don't see how we could be spinning on the >> waitqueue lock like that. I haven't seen any wbt bug reports like this >> before. >> >> Are things generally stable if you just turn off wbt? You can do that >> for sda, for instance, by doing: >> >> # echo 0 > /sys/block/sda/queue/wbt_lat_usec >> >> It'd be interesting to get this data point. Eg leave blk-mq enabled, and >> then just disable wbt. > > Hi Jens, > > Thanks for the speedy response. I'll see if I can get that tested soon; > if the system is stable without blk_mq I can see the users wanting to > keep it that way for a while. I'll let you know. Understandable. I just get suspicious of the general state of the system, if it's locking up there. Could be a hardware issue, or a bug in some other area that's messing things up. I have wbt running on literally hundreds of thousands of boxes and haven't seen a lockup like this. >> Is anything disabling wbt in the system otherwise? > > Not that I'm aware of, no. OK, just wanted to rule out something related to the shutdown path racing with IO. -- Jens Axboe
