On Thu, Oct 20, 2022 at 04:01:11PM -0400, Stefan Hajnoczi wrote: > On Thu, Oct 20, 2022 at 05:10:13PM +0800, Ming Lei wrote: > > Hi, > > > > David Jeffery found one double ->queue_rq() issue, so far it can > > be triggered in the following two cases: > > > > 1) scsi driver in guest kernel > > > > - the story could be long vmexit latency or long preempt latency of > > vCPU pthread, then IO req is timed out before queuing the request > > to hardware but after calling blk_mq_start_request() during ->queue_rq(), > > then timeout handler handles it by requeue, then double ->queue_rq() is > > caused, and kernel panic > > > > 2) burst of kernel messages from irq handler > > > > For 1), I think it is one reasonable case, given latency from host side > > can come anytime in theory because vCPU is emulated by one normal host > > pthread which can be preempted anywhere. For 2), I guess kernel message is > > supposed to be rate limited. > > > > Firstly, is this kind of so long(30sec) random latency when running kernel > > code something normal? Or do we need to take care of it? IMO, it looks > > reasonable in case of VM, but our VM experts may have better idea about this > > situation. Also the default 30sec timeout could be reduced via sysfs or > > drivers. > > 30 seconds is a long latency that does not occur during normal > operation, but unfortunately does happen on occasion. Thanks for the confirmation! > > I think there's an interest in understanding the root cause and solving > long latencies (if possible) in the QEMU/KVM communities. We can > investigate specific cases on kvm@xxxxxxxxxxxxxxx and/or > qemu-devel@xxxxxxxxxx. The issue was original reported on VMware VM, but maybe David can figure out how to trigger it on QEMU/KVM. Thanks, Ming
