Re: [PATCH] arm64: add NUMA emulation support

[Shuah Khan <shuah@kernel.org>]

On 09/11/2018 10:50 AM, Will Deacon wrote:
> On Tue, Sep 11, 2018 at 09:27:49AM -0600, Shuah Khan wrote:
>> On 09/11/2018 03:11 AM, Michal Hocko wrote:
>>> On Mon 10-09-18 20:02:05, Shuah Khan wrote:
>>>> Hi Michal,
>>>>
>>>> On 09/10/2018 07:48 AM, Michal Hocko wrote:
>>>>> On Fri 07-09-18 16:30:59, Shuah Khan wrote:
>>>>>> On 09/07/2018 02:34 AM, Michal Hocko wrote:
>>>>>>> On Thu 06-09-18 15:53:34, Shuah Khan wrote:
>>>> [....]
>>>>>>
>>>>>> In addition to isolation, being able to reserve a block instead is one of the
>>>>>> issues I am looking to address. Unfortunately memory cgroups won't address that
>>>>>> issue.
>>>>>
>>>>> Could you be more specific why you need reservations other than
>>>>> isolation.
>>>>>
>>>>
>>>> Taking automotive as a specific example, there are two classes of applications:
>>>> 1. critical applications that must run
>>>> 2. Infotainment and misc. user-space.
>>>>
>>>> In this case, being able to reserve a block of memory for critical applications
>>>> will ensure the memory is available for them. If a critical application has to
>>>> restart and/or when an on-demand critical application starts, it might not be able
>>>> to allocate memory if it is not reserved.
>>>>
>>>> When a flat system has multiple memory blocks, with NUMA emulation in conjunction with
>>>> cpusets, one or more block can be reserved for critical applications configuring a set
>>>> of cpus and one of more memory nodes for them.
>>>>
>>>> Memory cgroups will not support such reservation. Hope this helps explain the use-case
>>>> I am trying to address with this patch.
>>>
>>> OK, that is more clear. I still believe that you either have to have a
>>> very good control over memory allocations or a good luck to not see
>>> unexpected kernel allocations in your reserved memory which might easily
>>> break guarantees you would like to accomplish.
>>>
>>
>> Thanks. Right. I am with you on the possibility that root cgroup can eat into
>> the reserved memory. However, with this solution I proposed, there is a guarantee
>> that the cpuset cgroup that is configured for non-critical Infotainment and misc.
>> user-space application will not be able to allocate from the reserved memory node.
>>
>> I am hoping the proposed patch will allow critical apps. reserving memory with the
>> exception that root cgroup and kernel can still allocate from it when needed. Perhaps
>> cpuset exclusive logic could be extended to look for non-exclusive memory nodes first
>> if it doesn't already do that. This is inline with the current cpuset approach is that
>> the critical kernel allocations aren't starved to ensure memory reservations.
>>
>> If you don't think this solution isn't ideal/good, do you have other suggestions
>> for solving the problem? If not would it be okay to start with what I proposed and
>> build on top of as needed?
> 
> I still don't understand why this can't be achieved by faking up some NUMA
> entries in the firmware table and just using the existing NUMA code that we
> have.
> 

That is what is this patch is doing in some ways. Instead of hacking the firmware
tables, it provides a command line option to split the flat machine into specified
number of NUMA memory nodes.

In addition to the new config option and new command line handling, I added one
init routine that handles the NUMA emulation and after that normal NUMA code is
leveraged.

The only change is the following added to arm64_numa_init()

if (!numa_init(arm64_numa_emu_init))
+                       return;


arm64_numa_emu_init() does nothing unless the kernel is booted with the new
"numa=fake=N"

Please note that I am not adding any new NUMA code other than just this init
routine. When the command line is specified, instead of going down the
dummy_numa_init() path it will create NUMA emulation nodes.

I was very careful in identifying the minimal amount of code needed to add
this support. The change is limited to two existing routines:
numa_parse_early_param() and arm64_numa_init().

numa_init() which is common for all the variants including the fallback
dummy_numa_init()

This allows a cleaner way to split the memory and leverage all of the NUMA
code. This makes it easier to debug problems as opposed to hacked firmware
tables.

thanks,
-- Shuah