linux-kernel - Re: [PATCH] timekeeping: Align tick_sched_timer() with the HZ tick. -- regression report

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Message-ID: <1b2ed0be-7eb9-3224-5829-fcbdb00613e8@grsecurity.net>
Date:   Thu, 15 Jun 2023 13:40:30 +0200
From:   Mathias Krause <minipli@...ecurity.net>
To:     "Richard W.M. Jones" <rjones@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Gusenleitner Klaus <gus@...a.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] timekeeping: Align tick_sched_timer() with the HZ tick.
 -- regression report

On 15.06.23 11:03, Richard W.M. Jones wrote:
> On Wed, Jun 14, 2023 at 12:59:46AM +0200, Mathias Krause wrote:
>> This patch causes VM boot hangs for us. It took a while to identify as 
>> the boot hangs were only ~1 out of 30 but it's clearly it. Reverting 
>> the commit got me 100 boots in a row without any issue.
> 
> FWIW I have quite a nice test program for catching these sorts of boot
> hangs, see attached.  You need to change the VMLINUX define to point
> to your vmlinux or vmlinuz file.

Heh, nice. For this specific case, though, a simple boot loop in a
single VM was sufficient:

  $ kvm -cpu host -smp 8 -m 8G -display none -serial stdio -s \
        -kernel path/to/bzImage -append 'console=ttyS0 panic=1'

This will reboot the VM in an endless loop until it hangs which is
visually noticeable as there's no more output generated to the terminal.
At that time one can attach to it via gdb and investigate further:

  $ gdb -ex 'target remote :1234' path/to/vmlinux

Yet another advantage is that one can filter the output, e.g. via:

  $ kvm ... | grep clock

Thanks,
Mathias

> 
> Rich.
>