On Fri, Dec 01, 2017 at 02:46:39PM +0000, Ivan Kozik wrote:
> On Fri, Dec 1, 2017 at 1:35 PM, Ming Lei <ming.lei@redhat.com> wrote:
> > Basically all IO hang happen inside get_request(), and seems this issue
> > isn't related with recent change in V4.15, could you run V4.14 to see if
> > there is such issue?
> 
> I tried several times and I could not reproduce the problem on linus's v4.14.
> 
> Also, while testing, I noticed that udev rules can switch out the IO
> scheduler after gdisk writes partitions, but this appears to be
> unrelated to the problem, just something to remember when testing.


Now I can reproduce the issue on usb-storage.

After applying the attached debug patch, the log[1] can be generated.
>From the log, there are requests inserted to BFQ queue, but these requests
can't be retrieved by e->type->ops.mq.dispatch_request() in
blk_mq_do_dispatch_sched(), then IO hang is caused.

Not see this issue on mq-deadline, looks a BFQ specific issue.


[1] http://people.redhat.com/minlei/tests/logs/bfq-trace


-- 
Ming