Re: [PATCH v6] numa: make node_to_cpumask_map() NUMA_NO_NODE aware

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2019/9/24 19:28, Peter Zijlstra wrote:
> On Tue, Sep 24, 2019 at 07:07:36PM +0800, Yunsheng Lin wrote:
>> On 2019/9/24 17:25, Peter Zijlstra wrote:
>>> On Tue, Sep 24, 2019 at 09:29:50AM +0800, Yunsheng Lin wrote:
>>>> On 2019/9/24 4:34, Peter Zijlstra wrote:
>>>
>>>>> I'm saying the ACPI standard is wrong. Explain to me how it is
>>>>> physically possible to have a device without NUMA affinity in a NUMA
>>>>> system?
>>>>>
>>>>>  1) The fundamental interconnect is not uniform.
>>>>>  2) The device needs to actually be somewhere.
>>>>>
>>>>
>>>> From what I can see, NUMA_NO_NODE may make sense in the below case:
>>>>
>>>> 1) Theoretically, there would be a device that can access all the memory
>>>> uniformly and can be accessed by all cpus uniformly even in a NUMA system.
>>>> Suppose we have two nodes, and the device just sit in the middle of the
>>>> interconnect between the two nodes.
>>>>
>>>> Even we define a third node solely for the device, we may need to look at
>>>> the node distance to decide the device can be accessed uniformly.
>>>>
>>>> Or we can decide that the device can be accessed uniformly by setting
>>>> it's node to NUMA_NO_NODE.
>>>
>>> This is indeed a theoretical case; it doesn't scale. The moment you're
>>> adding multiple sockets or even board interconnects this all goes out
>>> the window.
>>>
>>> And in this case, forcing the device to either node is fine.
>>
>> Not really.
>> For packet sending and receiving, the buffer memory may be allocated
>> dynamically. Node of tx buffer memory is mainly based on the cpu
>> that is sending sending, node of rx buffer memory is mainly based on
>> the cpu the interrupt handler of the device is running on, and the
>> device' interrupt affinity is mainly based on node id of the device.
>>
>> We can bind the processes that are using the device to both nodes
>> in order to utilize memory on both nodes for packet sending.
>>
>> But for packet receiving, the node1 may not be used becuase the node
>> id of device is forced to node 0, which is the default way to bind
>> the interrupt to the cpu of the same node.
>>
>> If node_to_cpumask_map() returns all usable cpus when the device's node
>> id is NUMA_NO_NODE, then interrupt can be binded to the cpus on both nodes.
> 
> s/binded/bound/
> 
> Sure; the data can be allocated wherever, but the control structures are
> not dynamically allocated every time. They are persistent, and they will
> be local to some node.
> 
> Anyway, are you saying this stupid corner case is actually relevant?
> Because how does it scale out? What if you have 8 sockets, with each
> socket having 2 nodes and 1 such magic device. Then returning all CPUs
> is just plain wrong.

Yes, the hardware may not scale out, but what about the virtual device?

> 
>>>> 2) For many virtual deivces, such as tun or loopback netdevice, they
>>>> are also accessed uniformly by all cpus.
>>>
>>> Not true; the virtual device will sit in memory local to some node.
>>>
>>> And as with physical devices, you probably want at least one (virtual)
>>> queue per node.
>>
>> There may be similar handling as above for virtual device too.
> 
> And it'd be similarly broken.

From [1], there is a lot of devices with node id of NUMA_NO_NODE with the
FW_BUG.

[1] https://lore.kernel.org/lkml/5a188e2b-6c07-a9db-fbaa-561e9362d3ba@huawei.com/


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