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Date:   Fri, 06 Dec 2019 20:50:25 +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>
Subject: Re: Injecting delays into block layer
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Hello.

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?

I was playing with dm-delay in an isolated VM, but so far got no luck. 
I'll try to find another way to trigger this (if the bug is still 
present in 5.4) and get back to you in case of success.

For me it is a rare occurrence in production, and since I've upgraded to 
5.4 and disabled BFQ I haven't seen any at all. At this point I'm not 
even sure what I'm looking at. I'll try to re-enable BFQ soon to stress 
my production VMs again.

Thank you.

-- 
   Oleksandr Natalenko (post-factum)