linux-kernel - Re: + panic-avoid-the-extra-noise-dmesg.patch added to -mm tree

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Message-ID: <20181211082221.3wovowe6tql5o3s4@pathway.suse.cz>
Date:   Tue, 11 Dec 2018 09:22:21 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Feng Tang <feng.tang@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        akpm@...ux-foundation.org, bp@...e.de, keescook@...omium.org,
        mm-commits@...r.kernel.org, sergey.senozhatsky@...il.com,
        stable@...r.kernel.org, tglx@...utronix.de,
        Steven Rostedt <rostedt@...dmis.org>,
        Sasha Levin <sashal@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org
Subject: Re: + panic-avoid-the-extra-noise-dmesg.patch added to -mm tree

On Tue 2018-12-11 17:07:43, Sergey Senozhatsky wrote:
> On (12/10/18 16:57), Petr Mladek wrote:
> > > > (masked out) and on panic_cpu disables only SDEI (interrupts from firmware,
> > > > if I got it right); so it seems that arm64 can handle IRQs after panic. And
> > > > if there are platforms that handle IRQ (including sysrq) after panic, then
> > > > both options - making printk a noop or keeping local irqs off - maybe can
> > > > cause some problems. Or maybe not. We better ask arch people.
> > > 
> > > Yes, this is very valid concern. And after Petr and you raised it, I did
> > > some experiments with 3 x86 platforms at my hand, one Apollolake IOT device
> > > with serial console, one IvyBridge laptop and one Kabylake NUC, the magic key
> > > all works well before panic, and fails after panic. But I did remember the
> > > PageUp/PageDown key worked on some laptop years ago. And you actually raised a
> > > good question: what do we expect for the post-panic kernel?
> > 
> > I am not sure why it does not work. But it would be nice if sysrq
> > worked.
> 
> Absolutely.
> 
> [..]
> > I still think that calming down printk() is acceptable when
> > it can be restored from sysrq.
> 
> I would agree; peeking one of the two solutions, printk patch is
> probably preferable.
> 
> > I think that only few people might be interested into debugging
> > post-panic problems. We could print a warning for them about
> > that printk() has got disabled.
> 
> Dunno. This _maybe_ (speculation!) can upset folks on those platforms
> that have sysrq working after panic. printk is a common code.
> 
> I'm probably missing a lot of things here, but just in case, I'm not
> sure at which point the idea of patching some files under arch/x86
> directory was ruled out and why.

I suggested to clear the panic_blinking (or whatever name) in
__handle_sysrq(). The idea is that sysrq needs manual intervention.
It allows to see the original message before it gets overridden
by a potential sysrq-related output.

It assumes that sysrq is the only interesting operation when
printk() might be useful at this state.

Best Regards,
Petr