Message-ID: <d044b53b-4917-778d-0f77-c99da8f03769@huaweicloud.com>
Date: Mon, 26 Aug 2024 16:53:01 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Muchun Song <muchun.song@...ux.dev>, Yu Kuai <yukuai1@...weicloud.com>
Cc: Muchun Song <songmuchun@...edance.com>, Ming Lei <ming.lei@...hat.com>,
 Jens Axboe <axboe@...nel.dk>,
 "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH 2/4] block: fix ordering between checking BLK_MQ_S_STOPPED
 and adding requests to hctx->dispatch

Hi,

在 2024/08/26 16:35, Muchun Song 写道:
> 
> 
>> On Aug 22, 2024, at 11:54, Yu Kuai <yukuai1@...weicloud.com> wrote:
>>
>> Hi,
>>
>> 在 2024/08/19 11:49, Muchun Song 写道:
>>> On Mon, Aug 19, 2024 at 10:28 AM Ming Lei <ming.lei@...hat.com> wrote:
>>>>
>>>> Hi Muchun,
>>>>
>>>> On Sun, Aug 11, 2024 at 06:19:19PM +0800, Muchun Song wrote:
>>>>> Supposing the following scenario with a virtio_blk driver.
>>>>>
>>>>> CPU0                                                                CPU1
>>>>>
>>>>> blk_mq_try_issue_directly()
>>>>>      __blk_mq_issue_directly()
>>>>>          q->mq_ops->queue_rq()
>>>>>              virtio_queue_rq()
>>>>>                  blk_mq_stop_hw_queue()
>>>>>                                                                      virtblk_done()
>>>>>      blk_mq_request_bypass_insert()                                      blk_mq_start_stopped_hw_queues()
>>>>>          /* Add IO request to dispatch list */   1) store                    blk_mq_start_stopped_hw_queue()
>>>>>                                                                                  clear_bit(BLK_MQ_S_STOPPED)                 3) store
>>>>>      blk_mq_run_hw_queue()                                                       blk_mq_run_hw_queue()
>>>>>          if (!blk_mq_hctx_has_pending())                                             if (!blk_mq_hctx_has_pending())         4) load
>>>>>              return                                                                      return
>>>>>          blk_mq_sched_dispatch_requests()                                            blk_mq_sched_dispatch_requests()
>>>>>              if (blk_mq_hctx_stopped())          2) load                                 if (blk_mq_hctx_stopped())
>>>>>                  return                                                                      return
>>>>>              __blk_mq_sched_dispatch_requests()                                          __blk_mq_sched_dispatch_requests()
>>>>>
>>>>> The full memory barrier should be inserted between 1) and 2), as well as between
>>>>> 3) and 4) to make sure that either CPU0 sees BLK_MQ_S_STOPPED is cleared or CPU1
>>>>> sees dispatch list or setting of bitmap of software queue. Otherwise, either CPU
>>>>> will not re-run the hardware queue causing starvation.
>>>>
>>>> Yeah, it is one kind of race which is triggered when adding request into
>>>> ->dispatch list after returning STS_RESOURCE. We were troubled by lots of
>>>> such kind of race.
>>> Yes. I saw the similar fix for BLK_MQ_S_SCHED_RESTART.
>>>>
>>>> stopping queue is used in very less drivers, and its only purpose should
>>>> be for throttling hw queue in case that low level queue is busy. There seems
>>>> more uses of blk_mq_stop_hw_queues(), but most of them should be replaced
>>>> with blk_mq_quiesce_queue().
>>>>
>>>> IMO, fixing this kind of issue via memory barrier is too tricky to
>>>> maintain cause WRITE/READ dependency is very hard to follow. I'd suggest to
>>>> make memory barrier solution as the last resort, and we can try to figure
>>>> out other easier & more reliable way first.
>>> I do agree it is hard to maintain the dependencies in the future. We should
>>> propose an easy-maintainable solution. But I thought it is a long-term issue
>>> throughout different stable linux distros. Adding a mb is the easy way to fix
>>> the problem (the code footprint is really small), so it will be very
>>> easy for others
>>> to backport those bug fixes to different stable linux distros. Therefore, mb
>>> should be an interim solution. Then, we could improve it based on the solution
>>> you've proposed below. What do you think?
>>
>> I'll agree with Ming, let's figure out a better fix first. Easy to backport to stables is not first consideration.
> 
> Hi Kuai,
> 
> All right. I usually focus on MM, it seems there is a gap between MM and BLock.
> Anyway, let's figure out if there is any good solution.
> 
>>> Thanks,
>>> Muchun.
>>>>
>>>> One idea I thought of is to call blk_mq_request_bypass_insert()(or rename
>>>> & export it) before calling blk_mq_stop_hw_queue() in driver, then
>>>> return new status code STS_STOP_DISPATCH for notifying blk-mq to stop
>>>> dispatch simply.
>>
>> New status code look good to me, however, I wonder can we just remove
>> the problematic blk_mq_stop_hw_queue(), and replace it by handling the
>> new status from block layer?
>>
>> - Passing the new status to blk_mq_run_dispatch_ops, and quiesce with
> 
> I didn't fully understand your suggestion. Let me ask some questions.
> blk_mq_stop_hw_queue() is usually called in blk_mq_ops->queue_rq path,
> it'll be easy for this case to pass the new status to blk_mq_run_dispatch_ops.
> Should we remove blk_mq_stop_hw_queues() as well? How to pass the new
> status to blk_mq_run_dispatch_ops in this case?

For queue_rq from dispatch path, it can be removed. However, it is
called from remove path as well, I don't check yet if it can be removed
there, that's another story.

And just add a return value for dispatch_ops to pass status.

Thanks,
Kuai

> 
>> the new status, if no request is inflight, unquiesce immediately;
> 
> Actually, I didn't understand how to avoid the above race. May you elaborate
> the scenario?
> 
> Muhcun,
> Thanks.
> 
>> - unquiesce is any IO is done afterwards;
> 
> 
> 
> 
> .
> 

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