Re: [PATCH] powerpc/xmon: add debugfs entry for xmon

["Guilherme G. Piccoli" <gpiccoli@linux.vnet.ibm.com>]

On 14/02/2017 01:58, Pan Xinhui wrote:
> 
> 
> 在 2017/2/14 10:35, Nicholas Piggin 写道:
>> On Mon, 13 Feb 2017 19:00:42 -0200
>>
>> xmon state changing after the first sysrq+x violates principle of least
>> astonishment, so I think that should be fixed.
>>
> hi, Nick
> yes, as long as xmon is disabled during boot, it should still be disabled after existing xmon.
> My patch does not fix that as it need people add one more char 'z' following 'x'.
> I will provide a new patch to fix that.
> 
>> Then the question is, is it worth making it runtime configurable with xmon
>> command or debugfs tunables?
>>
> They are options for people to turn xmon features on or off. Maybe people needn't this.
> However I am not a fan  of debugfs this time as I am used to using xmon cmds. :)
> 
> Hi, Guilherme
> So in the end, my thought is that: 1) cmd x|X will exit xmon and keep xmon in the original state(indicated by var xmon_off).
> 2) Then add options to turn some features on/off. And debugfs maybe not fit for this. But I am also wondering at same time, are people needing this?

Hi Nick and Xinhui, thanks very much for the feedback.
I agree, we should keep xmon in the state it was firstly set, on boot
time - dropping to the debugger using sysrq shouldn't change it.

Now, the use case of the debugfs approach is to allow user to
enable/disable xmon without need to drop into the debugger itself, or
reboot the machine.
Imagine a scenario in which we have a production machine, and:


i) For some reason, somebody kept xmon enabled on grub.cfg and now, we
want to let kdump work in case of crash - how to disable xmon in runtime?

ii) The opposite: xmon wasn't enable on boot time in production machine,
but we have a super-rare issue and want to drop to xmon next time it
happens, so we need to enable it. But we don't want to drop into the
debugger to force it gets enabled, so how do we enable it?

Regarding the place of the xmon state file, I believe debugfs is the
right place - where else could we add it? procfs? configfs?

Thanks,


Guilherme
> 
> thanks
> xinhui
> 
>> Thanks,
>> Nick
>>
>