linux-kernel - Re: stalling IO regression since linux 5.12, through 5.18

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Message-Id: <7A67D5D9-EB63-4B0C-BC51-4A4CDBC2077E@linaro.org>
Date:   Tue, 6 Sep 2022 11:49:43 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     Jan Kara <jack@...e.cz>, Ming Lei <ming.lei@...hat.com>,
        Chris Murphy <lists@...orremedies.com>,
        Nikolay Borisov <nborisov@...e.com>,
        Jens Axboe <axboe@...nel.dk>,
        Btrfs BTRFS <linux-btrfs@...r.kernel.org>,
        Linux-RAID <linux-raid@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Josef Bacik <josef@...icpanda.com>,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: stalling IO regression since linux 5.12, through 5.18



> Il giorno 1 set 2022, alle ore 10:19, Yu Kuai <yukuai1@...weicloud.com> ha scritto:
> 
> 在 2022/09/01 16:03, Jan Kara 写道:
>> On Thu 01-09-22 15:02:03, Yu Kuai wrote:
>>> 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
>> And I guess:
>> 3. there are requests queued in other bfqqs
>> ?
> 
> Yes, of course, other bfqqs still have requests, but current
> implementation have flaws that even if other bfqqs doesn't have
> requests, bfq_asymmetric_scenario() can still return true because
> num_groups_with_pending_reqs > 0. We tried to fix this, however, there
> seems to be some misunderstanding with Paolo, and it's not applied to
> mainline yet...
> 

I think this is an unsolved performance issue (being solved patiently
by Yu Kuai), but not a functional flaw.  The solution of this issue
would probably solve this stall, but not the essential problem:
refcounting gets broken if reqs disappear for bfq without any
notification.

Thanks,
Paolo

> Thanks,
> Kuai
>> Otherwise I don't see a point in expiring current bfqq because there's
>> nothing bfq could do anyway. But under normal circumstances the request
>> completion should not take so long so I don't think it would be really
>> worth it to implement some special mechanism for this in bfq.
>> 								Honza