linux-kernel - Re: Removal of printk safe buffers delays NMI context printk

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Date:   Thu, 04 Nov 2021 17:24:28 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: Removal of printk safe buffers delays NMI context printk

Hi Nick,

On 2021-11-05, Nicholas Piggin <npiggin@...il.com> wrote:
> It seems printk from NMI context is now delayed indefinitely and
> there is no printk_safe_flush equivalent (or I can't see one) to
> allow a NMI buffer to be flushed by a different CPU.

NMI flushing is triggered using irq work (for the same CPU). This should
not have changed recently. Are you reporting a new issue?

> This causes hard lockup watchdog messages to not get shown on the
> console. I can call printk from a different CPU and that seems to
> flush the stuck CPU's NMI buffer immediately.

Perhaps we should be triggering the irq work on multiple CPUs if from
NMI context?

> What's the best way to expose this? Can we have something like tihs?
>
> void printk_flush(void)
> {
> 	preempt_disable();
> 	if (console_trylock_spinning())
> 		console_unlock();
> 	preempt_enable();
>         wake_up_klogd();
> }

We are planning on implementing a pr_flush() that will do something
similar. But I am wondering how you are planning on triggering a CPU to
call that function.

John Ogness