On Mon, Oct 24, 2022 at 11:30:39AM -0400, Stefan Hajnoczi wrote: > On Fri, Oct 21, 2022 at 10:23:57AM +0800, Ming Lei wrote: > > 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. > > A very basic question: > > The virtio_blk driver has no q->mq_ops->timeout() callback. Why does the > block layer still enable the timeout mechanism when the driver doesn't > implement ->timeout()? No matter if ->timeout() is implemented or not, request still may be timed out, and it is better for block layer to find such issue and simply reset timer in case of no ->timeout(). > > I saw there was some "idle" hctx logic and I guess the requests are timeout timer is reused for idle hctx detection. > resubmitted (although it wasn't obvious to me how that happens in the > code)? Maybe that's why the timer is still used if the driver doesn't > care about timeouts... Timeout handling is totally decided by driver's ->timeout() callback. If driver doesn't implement ->timeout(), the request's timer is reset. Thanks Ming
