Re: [PATCH RFC] sysrq: rcu-ify __handle_sysrq

Rik van Riel
2014-04-23T21:42:42Z

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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