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Date: Wed, 5 Dec 2018 14:29:12 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Feng Tang <feng.tang@intel.com>, Peter Zijlstra <peterz@infradead.org>
Cc: Petr Mladek <pmladek@suse.com>, akpm@linux-foundation.org, bp@suse.de,
	keescook@chromium.org, mm-commits@vger.kernel.org,
	sergey.senozhatsky@gmail.com, stable@vger.kernel.org,
	tglx@linutronix.de, Steven Rostedt <rostedt@goodmis.org>,
	Sasha Levin <sashal@kernel.org>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Andi Kleen <ak@linux.intel.com>
Subject: Re: + panic-avoid-the-extra-noise-dmesg.patch added to -mm tree
Message-ID: <20181205052912.GA423@jagdpanzerIV>
References: <20181204071531.z62N9NwXC%akpm@linux-foundation.org>
 <20181204102033.ltdvc7gmev2gvlkq@pathway.suse.cz>
 <20181204154936.wbgcovzpc54n6dvs@shbuild888>
 <20181205022654.GA503@jagdpanzerIV>
 <20181205024713.nqyt6qiamokq7qtl@shbuild888>
 <20181205025728.GC503@jagdpanzerIV>
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On (12/05/18 11:57), Sergey Senozhatsky wrote:
> On (12/05/18 10:47), Feng Tang wrote:
> > 
> > Btw, just FYI,  I just tried the sysrq (using minicom CTL + A + F + 'magic key'),
> > it works with system is running, but failed after I trigger a panic, I will
> > check more though I'm not very familiar with sysrq yet.

OK... So, apparently, what's happening is panic() calls smp_send_stop().
And smp_send_stop()->native_stop_other_cpus() on x86 disables local APIC.
So no fun anymore.


If I keep APIC enabled on panic CPU, then I have my keyboard working,
including PageUp-PageDown scrolling, sysrq handling, and so on.

I think I'm not the only one who'd want scrollback to work after panic
(yes, fremebuffer for debugging).


Andi Kleen [1] wrote (Cc-ed):
: Oops/warnings are getting longer and longer, often scrolling away
: from the screen, and if the kernel crashes backscroll does not work
: anymore, so precious information is lost.


PeterZ,
  for those folks who sometimes have to use framebuffer for debugging
  (just a trivial "let me scrollback and see the panic backtrace") and
  not always have access to serial console, can we have local APIC
  enabled on the panic_cpu? Or is it a terrible thing to ask for?


[1] https://lore.kernel.org/lkml/878tcvt592.fsf@linux.intel.com/T/#u

	-ss