linux-kernel - Re: [PATCH v3 4/4] printk/nmi: Increase the size of NMI buffer and make it configurable

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Message-ID: <20151218112902.GO6344@twins.programming.kicks-ass.net>
Date:	Fri, 18 Dec 2015 12:29:02 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Jiri Kosina <jikos@...nel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"adi-buildroot-devel@...ts.sourceforge.net" 
	<adi-buildroot-devel@...ts.sourceforge.net>,
	Cris <linux-cris-kernel@...s.com>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Linux-sh list <linux-sh@...r.kernel.org>,
	sparclinux <sparclinux@...r.kernel.org>
Subject: Re: [PATCH v3 4/4] printk/nmi: Increase the size of NMI buffer and
 make it configurable

On Fri, Dec 18, 2015 at 10:18:08AM +0000, Daniel Thompson wrote:
> I'm not entirely sure that this is an improvement.

What I do these days is delete everything in vprintk_emit() and simply
call early_printk().

Kill the useless kmsg buffer crap and locking, just pound bytes to the
UART registers without anything in between.

The other semi usable solution is redirecting to trace_printk() and
recovering the trace buffers from your kdump. But I've found that
typically kdump doesn't work anymore if you properly wedge the machine.
So this is very much a second rate solution.

But this globally locked buffer, calling out to console drivers that do
locking and even scheduling, is an unreliable unfixable trainwreck that
I've given up on.
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