linux-kernel - Re: NMIs reported by console

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Date: Thu, 4 Apr 2024 20:10:47 -0400
From: "John B. Wyatt IV" <jwyatt@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: John Ogness <john.ogness@...utronix.de>, Petr Mladek <pmladek@...e.com>,
	Clark Williams <williams@...hat.com>,
	Juri Lelli <jlelli@...hat.com>, Derek Barbosa <debarbos@...hat.com>,
	Bruno Goncalves <bgoncalv@...hat.com>,
	"John B. Wyatt IV" <sageofredondo@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: NMIs reported by console_blast.sh with 6.6.20-rt25

On Tue, Apr 02, 2024 at 12:34:14PM +0200, Sebastian Andrzej Siewior wrote:
> > Just before the sysrq that crashes the system.
> 
> so this is intentional.

Yes.

> > 
> > This is part of the console_blast.sh script that John Ogness sent me.
> > 
> > Please see below:
> …
> 
> Okay. Then everything works as it should…

Correct.

> 
> > > > NMI Backtrace for 6.6.20-rt25 no forced preemption with tuned throughput-performance profile
> > > > -----------------------------
> > > 
> > > This and the following backtrace shows the same picture: The CPU is
> > > crashing due to proc/sysrq request and does CPU-backtraces via NMI and
> > > polls in early_printk, waiting for the UART to become idle (probably).
> > > 
> > > I don't see an issue here so far.
> > 
> > Luis Goncalves discussed it with me after reading your response. Thank
> > you for your help. The NMI was needed to flush the buffers upon the
> > system crashing itself. Does this part about NMI watchdog need to be
> > documented?
> 
> Not sure about that one. There is an _a_ _lot_ to be printed from NMI
> and the NMI watchdog might trigger if nothing is triggering the
> NMI-watchdog during the print job. Also, the crash was requested.

I reran the 6.6 test and no NMI was reported with fully preemptive and
the realtime tuned profile. It was my error; my apologies for that.

I did include more of logs if you want to see here for my 6.8 testing.

https://lore.kernel.org/linux-rt-users/ZgWL2UyknaE2T70C@thinkpad2021/T/#u

Sincerely,
John Wyatt
Software Engineer, Core Kernel
Red Hat