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Date: Tue, 18 May 2010 21:47:23 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Xianghua Xiao <xiaoxianghua@gmail.com>
cc: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org
Subject: Re: PREEMPT_RT (2.6.33-rt17) disabled printk-to-console after
 console_init
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On Tue, 18 May 2010, Xianghua Xiao wrote:
> I could not get dmesg output to console/serial and
> klogd(/var/log/messages) anymore with 2.6.33-rt17 _after_ the kernel
> boot is done(on non-RT kernel it works fine). I made below changes it
> starts to work again, however I'm unsure how safe the change is.
> Please advise.

Well, obviously it's unsafe if you remove safety checks. And if you
care to look at the changelog of kernel/printk.c you'll find out why.

Short story: It's introducing nasty latencies.

Use "earlyprintk=serial,ttyS0,115200,keep" if you care, but do not
complain about the resulting latencies.

Thanks,

	tglx