Re: WARNING: CPU: 2 PID: 207 at drivers/nvme/host/core.c:527 nvme_setup_cmd+0x3d3

[Jens Axboe <axboe@kernel.dk>]

On 1/30/18 1:49 PM, Keith Busch wrote:
> On Tue, Jan 30, 2018 at 01:32:25PM -0700, Jens Axboe wrote:
>> On 1/30/18 1:30 PM, Keith Busch wrote:
>>> On Tue, Jan 30, 2018 at 08:57:49AM -0700, Jens Axboe wrote:
>>>>
>>>> Looking at the disassembly, 'n' is 2 and 'segments' is 0xffff.
>>>
>>> Is this still a problem if you don't use an IO scheduler? With deadline,
>>> I'm not finding any path to bio_attempt_discard_merge which is where the
>>> nr_phys_segments is supposed to get it set to 2. Not sure how it could
>>> becmoe 0xffff, though.
>>
>> blk_mq_make_request() -> blk_mq_sched_bio_merge() -> __blk_mq_sched_bio_merge()
>> 	-> blk_mq_attempt_merge() -> bio_attempt_discard_merge()
> 
> That's the calls only if you don't have an elevator_queue, right? With
> deadline, it looks like it goes through this path (ftrace confirms):
> 
>   __blk_mq_sched_bio_merge() -> dd_bio_merge() -> blk_mq_sched_try_merge()
> 
> Which doesn't have a case for ELEVATOR_DISCARD_MERGE.
> 
> Relavant function_graph:
> 
>   46)               |                    blk_mq_make_request() {
>   46)   0.133 us    |                      blk_queue_bounce();
>   46)   0.370 us    |                      blk_queue_split();
>   46)   0.314 us    |                      bio_integrity_prep();
>   46)   0.081 us    |                      blk_attempt_plug_merge();
>   46)               |                      __blk_mq_sched_bio_merge() {
>   46)               |                        dd_bio_merge() {
>   46)   0.792 us    |                          _raw_spin_lock();
>   46)               |                          blk_mq_sched_try_merge() {

Yeah I guess you are right, it can't happen for mq-deadline since the
generic sched path doesn't support it.

I'll see if I can make it happen again, but I'm not too hopeful. And if
I can't, it's hard to compare with "none" to see if it makes a difference
or not.

-- 
Jens Axboe