Re: stalling IO regression since linux 5.12, through 5.18

[Paolo Valente <paolo.valente@linaro.org>]

> Il giorno 1 set 2022, alle ore 09:02, Yu Kuai <yukuai1@huaweicloud.com> ha scritto:
> 
> Hi, Chris
> 
> 在 2022/08/20 15:00, Ming Lei 写道:
>> On Fri, Aug 19, 2022 at 03:20:25PM -0400, Chris Murphy wrote:
>>> 
>>> 
>>> On Thu, Aug 18, 2022, at 1:24 AM, Ming Lei wrote:
>>>> On Thu, Aug 18, 2022 at 12:27:04AM -0400, Chris Murphy wrote:
>>>>> 
>>>>> 
>>>>> On Thu, Aug 18, 2022, at 12:18 AM, Chris Murphy wrote:
>>>>>> On Thu, Aug 18, 2022, at 12:12 AM, Chris Murphy wrote:
>>>>>>> On Wed, Aug 17, 2022, at 11:41 PM, Ming Lei wrote:
>>>>>>> 
>>>>>>>> OK, can you post the blk-mq debugfs log after you trigger it on v5.17?
>>>>> 
>>>>> Same boot, 3rd log. But the load is above 300 so I kinda need to sysrq+b soon.
>>>>> 
>>>>> https://drive.google.com/file/d/1375H558kqPTdng439rvG6LuXXWPXLToo/view?usp=sharing
>>>>> 
>>>> 
>>>> Also please test the following one too:
>>>> 
>>>> 
>>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>>> index 5ee62b95f3e5..d01c64be08e2 100644
>>>> --- a/block/blk-mq.c
>>>> +++ b/block/blk-mq.c
>>>> @@ -1991,7 +1991,8 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx
>>>> *hctx, struct list_head *list,
>>>>  		if (!needs_restart ||
>>>>  		    (no_tag && list_empty_careful(&hctx->dispatch_wait.entry)))
>>>>  			blk_mq_run_hw_queue(hctx, true);
>>>> -		else if (needs_restart && needs_resource)
>>>> +		else if (needs_restart && (needs_resource ||
>>>> +					blk_mq_is_shared_tags(hctx->flags)))
>>>>  			blk_mq_delay_run_hw_queue(hctx, BLK_MQ_RESOURCE_DELAY);
>>>> 
>>>>  		blk_mq_update_dispatch_busy(hctx, true);
>>>> 
>>> 
>>> 
>>> With just this patch on top of 5.17.0, it still hangs. I've captured block debugfs log:
>>> https://drive.google.com/file/d/1ic4YHxoL9RrCdy_5FNdGfh_q_J3d_Ft0/view?usp=sharing
>> The log is similar with before, and the only difference is RESTART not
>> set.
>> Also follows another patch merged to v5.18 and it fixes io stall too, feel free to test it:
>> 8f5fea65b06d blk-mq: avoid extending delays of active hctx from blk_mq_delay_run_hw_queues
> 
> Have you tried this patch?
> 
> We meet a similar problem in our test, and I'm pretty sure about the
> situation at the scene,
> 
> Our test environment：nvme with bfq ioscheduler,
> 
> How io is stalled:
> 
> 1. hctx1 dispatch rq from bfq in service queue, bfqq becomes empty,
> dispatch somehow fails and rq is inserted to hctx1->dispatch, new run
> work is queued.
> 
> 2. other hctx tries to dispatch rq, however, in service bfqq is
> empty, bfq_dispatch_request return NULL, thus
> blk_mq_delay_run_hw_queues is called.
> 
> 3. for the problem described in above patch，run work from "hctx1"
> can be stalled.
> 
> Above patch should fix this io stall, however, it seems to me bfq do
> have some problems that in service bfqq doesn't expire under following
> situation:
> 
> 1. dispatched rqs don't complete
> 2. no new rq is issued to bfq
> 

There may be one more important problem: is bfq_finish_requeue_request
eventually invoked for the failed rq?  If it is not, then a memory
leak follows, because recounting gets unavoidably unbalanced.

In contrast, if bfq_finish_requeue_request is correctly invoked, then
no stall should occur.

Thanks,
Paolo

> Thanks,
> Kuai
>> Thanks,
>> Ming
>> .