linux-kernel - Re: stalling IO regression since linux 5.12, through 5.18

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Date:   Fri, 12 Aug 2022 12:05:12 -0400
From:   "Chris Murphy" <lists@...orremedies.com>
To:     "Josef Bacik" <josef@...icpanda.com>, paolo.valente@...aro.org
Cc:     "Btrfs BTRFS" <linux-btrfs@...r.kernel.org>,
        Linux-RAID <linux-raid@...r.kernel.org>,
        linux-block@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: stalling IO regression since linux 5.12, through 5.18

On Wed, Aug 10, 2022, at 3:34 PM, Chris Murphy wrote:
> Booted with cgroup_disable=io, and confirmed cat 
> /sys/fs/cgroup/cgroup.controllers does not list io.

The problem still reproduces with the cgroup IO controller disabled.

On a whim, I decided to switch the IO scheduler from Fedora's default bfq for rotating drives to mq-deadline. The problem does not reproduce for 15+ hours, which is not 100% conclusive but probably 99% conclusive. I then switched live while running the workload to bfq on all eight drives, and within 10 minutes the system cratered, all new commands just hang. Load average goes to triple digits, i/o wait increasing, i/o pressure for the workload tasks to 100%, and IO completely stalls to zero. I was able to switch only two of the drive queues back to mq-deadline and then lost responsivness in that shell and had to issue sysrq+b...

Before that I was able to extra sysrq+w and sysrq+t.
https://drive.google.com/file/d/16hdQjyBnuzzQIhiQT6fQdE0nkRQJj7EI/view?usp=sharing

I can't tell if this is a bfq bug, or if there's some negative interaction between bfq and scsi or megaraid_sas. Obviously it's rare because otherwise people would have been falling over this much sooner. But at this point there's strong correlation that it's bfq related and is a kernel regression that's been around since 5.12.0 through 5.18.0, and I suspect also 5.19.0 but it's being partly masked by other improvements.

-- 
Chris Murphy