linux-kernel - Re: bug in tag handling in blk-mq?

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Date:   Wed, 9 May 2018 11:01:52 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Mike Galbraith <efault@....de>,
        Paolo Valente <paolo.valente@...aro.org>
Cc:     Christoph Hellwig <hch@....de>,
        linux-block <linux-block@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>
Subject: Re: bug in tag handling in blk-mq?

On 5/9/18 10:57 AM, Mike Galbraith wrote:
> On Wed, 2018-05-09 at 09:18 -0600, Jens Axboe wrote:
>> On 5/8/18 10:11 PM, Mike Galbraith wrote:
>>> On Tue, 2018-05-08 at 19:09 -0600, Jens Axboe wrote:
>>>>
>>>> Alright, I managed to reproduce it. What I think is happening is that
>>>> BFQ is limiting the inflight case to something less than the wake
>>>> batch for sbitmap, which can lead to stalls. I don't have time to test
>>>> this tonight, but perhaps you can give it a go when you are back at it.
>>>> If not, I'll try tomorrow morning.
>>>>
>>>> If this is the issue, I can turn it into a real patch. This is just to
>>>> confirm that the issue goes away with the below.
>>>
>>> Confirmed.  Impressive high speed bug stomping.
>>
>> Well, that's good news. Can I get you to try this patch?
> 
> Sure thing.  The original hang (minus provocation patch) being
> annoyingly non-deterministic, this will (hopefully) take a while.

You can verify with the provocation patch as well first, if you wish.
Just need to hand-apply since it'll conflict with this patch in
bfq. But it's a trivial resolve.

-- 
Jens Axboe