linux-kernel - Re: [PATCH rcu 11/12] torture: Flush printk() buffers before powering off

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Message-ID: <87ilotagdw.fsf@jogness.linutronix.de>
Date:   Tue, 21 Jun 2022 22:58:27 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org
Subject: Re: [PATCH rcu 11/12] torture: Flush printk() buffers before
 powering off

On 2022-06-21, "Paul E. McKenney" <paulmck@...nel.org> wrote:
> The patch below will cause rcutorture to implicitly test this
> functionality, unless told otherwise, for example, by using the
> --bootargs "torture.printk_shutdown_bug_workaround" kvm.sh
> argument.
>
> Thoughts?

I feel like this is dirtying the torture.* bootarg namespace a
bit. Also, I am not sure how useful it is as a dynamic option. I assume
that users would generally avoid using it, so its very existence might
just be more noise in the documentation and code. It is an unusual
feature:

"In case some bug shows up, here is a flag to avoid it."

I personally would just drop the patch and rely on a correctly
functional kernel. But I am also not an rcutorture user. If _you_ think
that such a flag is useful, feel free to include the patch.

> commit 204bf1e2a5a2fb68c15b4b64793ad0896db6f705
> Author: Paul E. McKenney <paulmck@...nel.org>
> Date:   Tue Jun 21 11:02:25 2022 -0700
>
>     torture: Optionally flush printk() buffers before powering off
>     
>     The rcutorture test suite produces quite a bit of console output at
>     the end of a test.  This means that the new-in-2022 printk() kthreads
>     are likely to be in the process of flushing output at the time of the
>     torture_shutdown() function's call to kernel_power_off().  Normally,
>     rcutorture relies on printk() to flush any pending output upon shutdown,
>     the better to detect bugs in this area, for example, the one introduced
>     by 8e274732115f ("printk: extend console_lock for per-console locking").
>     However, once such a bug is detected and reported, it is necessary to
>     test the rest of the system, without noise from the already-reported bug.
>     
>     This commit therefore adds a torture.printk_shutdown_bug_workaround
>     kernel parameter, which causes torture_shutdown() to invoke pr_flush(),
>     and print an informative message on the console, immediately before
>     invoking kernel_power_off().  When this kernel parameter is not specified,
>     it is up to printk() to flush its own buffers.
>     
>     Suggested-by: John Ogness <john.ogness@...utronix.de>
>     Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

Reviewed-by: John Ogness <john.ogness@...utronix.de>