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Subject: Re: bug in tag handling in blk-mq?
From: Mike Galbraith <efault@gmx.de>
To: Jens Axboe <axboe@kernel.dk>, Paolo Valente <paolo.valente@linaro.org>
Cc: Christoph Hellwig <hch@lst.de>,
        linux-block <linux-block@vger.kernel.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Oleksandr Natalenko <oleksandr@natalenko.name>
Date: Wed, 09 May 2018 18:57:10 +0200
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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.

	-Mike