linux-kernel - printk.time causes rare kernel boot hangs

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Message-ID: <20230613134105.GA10301@redhat.com>
Date:   Tue, 13 Jun 2023 14:41:05 +0100
From:   "Richard W.M. Jones" <rjones@...hat.com>
To:     Aaron Thompson <dev@...ont.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org
Subject: printk.time causes rare kernel boot hangs

[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@...ont.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.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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