linux-kernel - Re: [PATCH v4] Documentation, dt, numa: Add note to empty NUMA node

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Message-ID: <62d04903-e4f3-2302-d530-5942e33ae116@redhat.com>
Date:   Fri, 25 Jun 2021 21:14:49 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     Andrew Jones <drjones@...hat.com>
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        rdunlap@...radead.org, robh+dt@...nel.org, shan.gavin@...il.com
Subject: Re: [PATCH v4] Documentation, dt, numa: Add note to empty NUMA node

On 6/25/21 8:18 PM, Andrew Jones wrote:
> On Fri, Jun 25, 2021 at 06:36:48PM +1000, Gavin Shan wrote:
>> On 6/25/21 5:06 PM, Andrew Jones wrote:
>>> On Fri, Jun 25, 2021 at 09:02:17AM +0200, Andrew Jones wrote:
>>>> On Fri, Jun 25, 2021 at 01:23:38PM +0800, Gavin Shan wrote:
>>>>> The empty memory nodes, where no memory resides in, are allowed.
>>>>> For these empty memory nodes, the 'len' of 'reg' property is zero.
>>>>> The NUMA node IDs are still valid and parsed, but memory can be
>>>>> added to them through hotplug afterwards. I finds difficulty to
>>>>> get where it's properly documented.
>>>>>
>>>>> So lets add note to empty memory nodes in the NUMA binding doc.
>>>>>
>>>>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>>>>> ---
>>>>>    Documentation/devicetree/bindings/numa.txt | 4 ++++
>>>>>    1 file changed, 4 insertions(+)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/numa.txt b/Documentation/devicetree/bindings/numa.txt
>>>>> index 21b35053ca5a..0fc882e44270 100644
>>>>> --- a/Documentation/devicetree/bindings/numa.txt
>>>>> +++ b/Documentation/devicetree/bindings/numa.txt
>>>>> @@ -109,6 +109,10 @@ Example:
>>>>>    Dual socket system consists of 2 boards connected through ccn bus and
>>>>>    each board having one socket/soc of 8 cpus, memory and pci bus.
>>>>> +Note that empty memory nodes, which no memory resides in, are allowed.
>>>>> +The NUMA node IDs in these empty memory nodes are still valid, but
>>>>> +memory can be added into them through hotplug afterwards.
>>>>
>>>> Please change the second sentence to:
>>>>
>>>>     The NUMA node IDs in these empty memory nodes are still valid and
>>>>     memory may be added into them through hotplug afterwards.
>>>>
>>>> But, this doesn't look like the right place for this paragraph. You're
>>>> adding the paragraph to the example section, but the example doesn't have
>>>> any empty memory nodes.
>>>>
>>>> I think the paragraph should be added to section "2 - numa-node-id" and an
>>>
>>> Or maybe even create a new section for it.
>>>
>>>> example empty memory node should be provided. Also, the commit message
>>>> talks about the length of 'reg' being zero, which is an important
>>>> distinction which should also be documented.
>>>>
>>
>> Drew, thanks for your comments. Yeah, it sounds sensible to create
>> a new section for it and an example would be more helpful. Please
>> check if below changes are fine to you. I probably need Randy's review
>> again.
>>
>> I'm trying to avoid too many revisions for this sort of trivial patch,
>> even though I already had. However, it's time frame for v5.14 and I'm
>> pushing this to be merged during the cycle.
> 
> We must revise until we're satisfied... Also, I wouldn't call writing
> specifications trivial.
> 

[...]

> I'll just rewrite it:
> 
>   Empty memory nodes, which no memory resides in, are allowed. The 'length'
>   field of the 'reg' property is zero, but the 'base-address' is a dummy
>   address and invalid. However, the NUMA node IDs and distance maps are
>   still valid and memory may be added into them through hotplug afterwards.
> 

[...]

> 
> You should write how you've selected the dummy addresses for the empty
> memory nodes, perhaps here in the /* Empty memory node */ comments or
> above in the paragraph or both.
> 
>> +	memory@...00000000 {
>> +		device_type = "memory";
>> +		reg = <0x1 0x0 0x0 0x0>;
>> +		numa-node-id = <2>;
>> +	};
>> +
>> +	/* Empty memory node */
>> +	memory@...80000000 {
>> +		device_type = "memory";
>> +		reg = <0x1 0x80000000 0x0 0x0>;
>> +		numa-node-id = <3>;
>> +	};
>> +

[...]

Thanks, Drew. As 'base-address' is dummy address and invalid. So we
would note it as suggestion. With your suggestion integrated, how about
something like this?

==============================================================================
4 - Empty memory nodes
==============================================================================

Empty memory nodes, which no memory resides in, are allowed. The 'length'
field of the 'reg' property is zero, but the 'base-address' is a dummy
address and invalid. The 'base-address' could be the summation of highest
memory address plus the NUMA node ID. However, the NUMA node IDs and
distance maps are still valid and memory may be added into them through
hotplug afterwards.

        :

        memory@0 {
                 device_type = "memory";
                 reg = <0x0 0x0 0x0 0x80000000>;
                 numa-node-id = <0>;
         };

         memory@...0000000 {
                 device_type = "memory";
                 reg = <0x0 0x80000000 0x0 0x80000000>;
                 numa-node-id = <1>;
         };

         /* Empty memory node */
         memory@...00000002 {
                 device_type = "memory";
                 reg = <0x1 0x2 0x0 0x0>;
                 numa-node-id = <2>;
         };

         /* Empty memory node */
         memory@...00000003 {
                 device_type = "memory";
                 reg = <0x1 0x3 0x0 0x0>;
                 numa-node-id = <3>;
         };

         :

Thanks,
Gavin