Re: [PATCH 1/2] numa: Add simple generic NUMA emulation

[Tvrtko Ursulin <tvrtko.ursulin@igalia.com>]

Hi Greg,

On 26/06/2024 08:38, Greg Kroah-Hartman wrote:
> On Tue, Jun 25, 2024 at 01:58:02PM +0100, Tvrtko Ursulin wrote:
>> From: Maíra Canal <mcanal@igalia.com>
>>
>> Add some common code for splitting the memory into N emulated NUMA memory
>> nodes.
>>
>> Individual architecture can then enable selecting this option and use the
>> existing numa=fake=<N> kernel argument to enable it.
>>
>> Memory is always split into equally sized chunks.
>>
>> Signed-off-by: Maíra Canal <mcanal@igalia.com>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
>> Co-developed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Cc: “Rafael J. Wysocki" <rafael@kernel.org>
>> ---
>>   drivers/base/Kconfig          |  7 ++++
>>   drivers/base/Makefile         |  1 +
>>   drivers/base/arch_numa.c      |  6 ++++
>>   drivers/base/numa_emulation.c | 67 +++++++++++++++++++++++++++++++++++
>>   drivers/base/numa_emulation.h | 21 +++++++++++
> 
> Why not just properly describe the numa topology in your bootloader or
> device tree and not need any such "fake" stuff at all?
> 
> Also, you are now asking me to maintain these new files, not something
> I'm comfortable doing at all sorry.

Mostly because ae3c107cd8be ("numa: Move numa implementation to common 
code") and existing common code in drivers/base/arch_numa.c it appeared 
it could be acceptable to add the simple NUMA emulation into the common 
code too. Then building upon the same concept as on x86 where no need 
for firmware changes is needed for experimenting with different 
configurations.

Would folding into arch_numa.c so no new files are added address your 
concern, or your main issue is the emulation in general?

 >> +	if (str_has_prefix(opt, "fake="))
 >> +		return numa_emu_cmdline(opt + 5);
 >
 > You did not document this at all :(

That was indeed an oversight. Just need to "copy with edits" some stuff 
from Documentation/arch/x86/x86_64/boot-options.rst.

Regards,

Tvrtko