Re: [Patch 6/8] powerpc: add CONFIG_KEXEC_AUTO_RESERVE

["M. Mohan Kumar" <mohan@in.ibm.com>]

On Tue, Aug 25, 2009 at 02:23:04PM +0800, Amerigo Wang wrote:
> Michael Ellerman wrote:
>> On Fri, 2009-08-21 at 02:55 -0400, Amerigo Wang wrote:
>>   
>>> Introduce a new config option KEXEC_AUTO_RESERVE for powerpc.
>>>
>>> Index: linux-2.6/arch/powerpc/Kconfig
>>> =================================>>> --- linux-2.6.orig/arch/powerpc/Kconfig
>>> +++ linux-2.6/arch/powerpc/Kconfig
>>> @@ -346,6 +346,17 @@ config KEXEC
>>>  	  support.  As of this writing the exact hardware interface is
>>>  	  strongly in flux, so no good recommendation can be made.
>>>  +config KEXEC_AUTO_RESERVE
>>> +	bool "automatically reserve memory for kexec kernel"
>>> +	depends on KEXEC
>>> +	default y
>>> +	---help---
>>> +	  Automatically reserve memory for a kexec kernel, so that you don't
>>> +	  need to specify numbers for the "crashkernel=X@Y" boot option,
>>> +	  instead you can use "crashkernel=auto". To make this work, you need
>>> +	  to have more than 4G memory. On PPC, 256M is reserved, 1/32 memory
>>> +	  on PPC64, but it will not exceed 1T/32.
>>>     
>>
>> To be honest I don't see why this logic goes in the kernel. It seems to
>> me that it's policy how much memory you devote to the crash kernel vs
>> the production kernel. It depends on what kind of crash kernel you're
>> loading, a minimal UP dump kernel, or a full-featured SMP behemoth, An
>> it depends on how much memory you're willing to leave idle in the
>> off-chance you crash.
>>   
>
> True, but since in the crash kernel, we have very little memory, so  
> probably loading a full-featured SMP kernel doesn't make much sense...
>
> And in patch 1/8, I introduced a way to free the reserved memory at  
> run-time.
>
>> That aside, I don't see how this will be useful in practice, if it only
>> works for memory sizes over 4G? Or are we saying that people with less
>> than 4G don't need crash kernels? If we're not saying that, those users,
>> or those users' distros, still need to do some logic to work out if they
>> have < 4GB of memory and if so pick a crash kernel size. So why can't
>> they pick the size in the > 4GB case also?
>>   
>
> No, we set 4G as a threshold because we only want this work when have  
> have enough memory which is defined as 4G currently... This can be  
> changed to arch-dependent, e.g. ppc. I am very open to this.
>

So the distro/admin have to use crashkernel=auto for machines having more
than 4GB RAM and for machines with less than 4GB RAM they have to use the
crashkernel=x@y (or extended crashkernel syntax)? IMHO it will be nice if
crashkernel=auto could handle all of the situations.

Regards,
M. Mohan Kumar