On 12/05/17 06:16, Ming Lei wrote:

On Mon, Dec 04, 2017 at 11:48:07PM +0000, Holger Hoffstätte wrote:

quoted

On Tue, 05 Dec 2017 06:45:08 +0800, Ming Lei wrote:

quoted

On Mon, Dec 04, 2017 at 03:09:20PM +0000, Bart Van Assche wrote:

quoted

On Sun, 2017-12-03 at 00:31 +0800, Ming Lei wrote:

quoted

Fixes: 0df21c86bdbf ("scsi: implement .get_budget and .put_budget for blk-mq")

It might be safer to revert commit 0df21c86bdbf instead of trying to fix all
issues introduced by that commit for kernel version v4.15 ...

What are all issues in v4.15-rc? Up to now, it is the only issue reported,
and can be fixed by this simple patch, which one can be thought as cleanup
too.

Even with this patch I've encountered at least one hang that
seemed related. I'm using most of block/scsi-4.15 on top of 4.14 and
the hang in question was on a rotating disk. It could be solved by activating
a different scheduler on the hanging device; all hanging sync/df processes got
unstuck and all was fine again, which leads me to believe that there is at least
one more rare condition where delaying requests (as done in the budget patch)
leads to a hang.

This happened with mq-deadline which I was testing specifically to avoid
any BFQ-related side effects.

OK, this looks a new report.

Without any log, we can't make any progress, and even we can't guess
what the issue is related with.

Considering that you just had an idea about a corner case and posted v2
of the patch, it's safe to say that we actually can...which is why I
described the situation exactly the way I did. :)

I did try to get stack traces but all the hanging processes were
simply stuck on the device mutex (deep inside btrfs), so nothing too
helpful.

Could you post your dmesg log(include the hang process stack trace)? And
dump the debugfs log by the following script when this hang happens?

	http://people.redhat.com/minlei/tests/tools/dump-blk-info

BTW, you just need to pass the disk name to the script, such as: /dev/sda.

Thanks for the script. I'm now running with the new patch and will see what
happens.

cheers,
Holger