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Date:   Mon, 23 Dec 2019 01:10:20 +0100
From:   Oleksandr Natalenko <oleksandr@natalenko.name>
To:     Paolo Valente <paolo.valente@linaro.org>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        linux-block <linux-block@vger.kernel.org>,
        SIMONE RICHETTI <206161@studenti.unimore.it>, tytso@mit.edu
Subject: Re: Injecting delays into block layer
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Hi.

On 06.12.2019 17:17, Paolo Valente wrote:
> Simone (in CC) and I have worked a little bit on reproducing the I/O
> freeze you report.  Simone made a small change in SCSI_debug, which
> makes the latter serve I/O with a highly varying random delay (100ms -
> 1s), about twice a second.
> 
> Then, to generate some fluctuating and heavy I/O, he ran the
> comm_startup_lat.sh script of my S suite with SCSI_debug a few times.
> Unfortunately, he didn't succeed in reproducing the problem.  If you
> want, we can send you a patch with his change for SCSI_debug.
> 
> Any news on your side?

FWIW, I guess I'm safe to exclude BFQ at the moment since I've 
encountered a very similar issue without having BFQ enabled.

Also, I think this might be unrelated to the block layer at all. I 
suspect there's some race between MADV_MERGEABLE and MADV_DONTNEED since 
this is what's hammering the affected tasks and what I see from the call 
traces.

I'll investigate further and probably talk to MM people instead. Sorry 
for the noise.

-- 
   Oleksandr Natalenko (post-factum)