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Date:   Sun, 18 Jun 2023 12:25:35 +0200
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Subject: Re: printk.time causes rare kernel boot hangs
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From:   "Linux regression tracking #update (Thorsten Leemhuis)" 
        <regressions@leemhuis.info>
To:     "Richard W.M. Jones" <rjones@redhat.com>,
        Aaron Thompson <dev@aaront.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org,
        Linux kernel regressions list <regressions@lists.linux.dev>
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          Linux regressions mailing list 
          <regressions@lists.linux.dev>
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On 13.06.23 16:07, Linux regression tracking #adding (Thorsten Leemhuis)
wrote:
> 
> On 13.06.23 15:41, Richard W.M. Jones wrote:
>> [Being tracked in this bug which contains much more detail:
>> https://gitlab.com/qemu-project/qemu/-/issues/1696 ]
>>
>> Recent kernels hang rarely when booted on qemu.  Usually you need to
>> boot 100s or 1,000s of times to see the hang, compared to 292,612 [sic]
>> successful boots which I was able to do before the problematic commit.
>>
>> A reproducer (you'll probably need to use Fedora) is:
>>
>>   $ while guestfish -a /dev/null -v run >& /tmp/log; do echo -n . ; done
>>
>> You will need to leave it running for probably several hours, and
>> examine the /tmp/log file at the end.
>>
>> I tracked this down to the following commit:
>>
>>   commit f31dcb152a3d0816e2f1deab4e64572336da197d
>>   Author: Aaron Thompson <dev@aaront.org>
>>   Date:   Thu Apr 13 17:50:12 2023 +0000
>>
>>     sched/clock: Fix local_clock() before sched_clock_init()
>>     
>>     Have local_clock() return sched_clock() if sched_clock_init() has not
>>     yet run. sched_clock_cpu() has this check but it was not included in the
>>     new noinstr implementation of local_clock().
>>
>>   (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f31dcb152a3d0816e2f1deab4e64572336da197d)
>>
>> Reverting this commit fixes the problem.
>>
>> I don't know _why_ this commit is wrong, but can we revert it as it
>> causes serious problems with libguestfs hanging randomly.
>>
>> Or if there's anything you want me to try out then let me know,
>> because I can reproduce the problem locally quite easily.
> 
> Thanks for the report. To be sure the issue doesn't fall through the
> cracks unnoticed, I'm adding it to regzbot, the Linux kernel regression
> tracking bot:
> 
> #regzbot ^introduced f31dcb152a3d0816e2f1deab4e64572336da197d
> #regzbot title sched/clock: printk.time causes rare kernel boot hangs
> #regzbot ignore-activity

#regzbot fix: tick/common: Align tick period during sched_timer setup
#regzbot monitor:
https://lore.kernel.org/all/12c6f9a3-d087-b824-0d05-0d18c9bc1bf3@amazon.com/
#regzbot ignore-activity

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
Everything you wanna know about Linux kernel regression tracking:
https://linux-regtracking.leemhuis.info/about/#tldr
That page also explains what to do if mails like this annoy you.