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:

quoted

On Thu, Oct 20, 2022 at 05:10:13PM +0800, Ming Lei wrote:

quoted

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!

quoted

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@vger.kernel.org and/or
qemu-devel@nongnu.org.

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()?

I saw there was some "idle" hctx logic and I guess the requests are
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...

Stefan