linux-kernel - Re: [PATCH RFC] sysrq: rcu-ify __handle

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Message-ID: <alpine.LRH.2.00.1404232338200.1491@twin.jikos.cz>
Date:	Wed, 23 Apr 2014 23:39:43 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	joern@...fs.org, peterz@...radead.org, cxie@...hat.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH RFC] sysrq: rcu-ify __handle_sysrq

On Wed, 23 Apr 2014, Andrew Morton wrote:

> > Echoing values into /proc/sysrq-trigger seems to be a popular way to
> > get information out of the kernel. However, dumping information about
> > thousands of processes, or hundreds of CPUs to serial console can
> > result in IRQs being blocked for minutes, resulting in various kinds
> > of cascade failures.
> > 
> > The most common failure is due to interrupts being blocked for a very
> > long time. This can lead to things like failed IO requests, and other
> > things the system cannot easily recover from.
> 
> I bet nobody wants that console output anyway.  You do the sysrq then
> run dmesg or look in /var/log/messages to see what happened.  People
> who are experiencing problems such as this should run `dmesg -n 1'
> before writing to sysrq-trigger.

I don't agree. I have used sysrq-t multiple times in situations where 
userspace was already dead, but sysrq was still able to provide valuable 
information about the state of the kernel.

-- 
Jiri Kosina
SUSE Labs

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