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

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Message-Id: <20140423144935.a024c26edca9c2c46c3036a7@linux-foundation.org>
Date:	Wed, 23 Apr 2014 14:49:35 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jiri Kosina <jkosina@...e.cz>
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 23:44:47 +0200 (CEST) Jiri Kosina <jkosina@...e.cz> wrote:

> 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.
> > > 
> > 
> > I'm talking about /proc/sysrq-trigger, not the magic key combo.
> 
> At the end of the day, that reaches the same __handle_sysrq() codepath, 
> no?

What I'm proposing is that we provide a way in which console output may be
suppressed during /proc/sysrq-trigger writes.  I'm not suggesting that the
implementation be buggy ;)

But clueful people can run `dmesg -1' beforehand, so I wonder if the
patch really has much value?
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