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

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Message-ID: <535833AE.6010505@redhat.com>
Date:	Wed, 23 Apr 2014 17:42:06 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Jiri Kosina <jkosina@...e.cz>
CC:	linux-kernel@...r.kernel.org, joern@...fs.org,
	peterz@...radead.org, akpm@...ux-foundation.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 04/23/2014 05:37 PM, Jiri Kosina wrote:
> On Wed, 23 Apr 2014, Rik van Riel 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.
>>
>> This problem is easily fixable by making __handle_sysrq use RCU
>> instead of spin_lock_irqsave.
>>
>> This leaves the warning that RCU grace periods have not elapsed for a
>> long time, but the system will come back from that automatically.
> 
> This, however, will make RCU stall detector to send NMI to all online CPUs 
> so that they can dump their stacks.

It already does that, since several of the longer-running
sysrq handlers already grab rcu_read_lock(), for example
show_state().

> IOW, this might actually make the whole sysrq dump last for much longer, 
> and have the log polluted with all-CPU dumps for no good reason.
> 
> I wonder whether explicitly setting rcu_cpu_stall_suppress during sysrq 
> handling might be a viable workaround for this.

I suppose that would do the trick.

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