RE: [RFC PATCH 1/2] sched/topology: Get rid of NUMA overlapping groups

[Valentin Schneider <valentin.schneider@arm.com>]

On 09/02/21 00:12, Song Bao Hua (Barry Song) wrote:
>> -----Original Message-----
>> From: Valentin Schneider [mailto:valentin.schneider@arm.com]
>>
>> Yes; let's take your topology for instance:
>>
>> node   0   1   2   3
>>     0:  10  12  20  22
>>     1:  12  10  22  24
>>     2:  20  22  10  12
>>     3:  22  24  12  10
>>
>>       2       10      2
>>   0 <---> 1 <---> 2 <---> 3
>
> Guess you actually mean
>        2       10      2
>    1 <---> 0 <---> 2 <---> 3
>

Yeah, you're right, sorry about that!

>>
>>
>> Domains for node1 will look like (before any fixes are applied):
>>
>> NUMA<=10: span=1   groups=(1)
>> NUMA<=12: span=0-1 groups=(1)->(0)
>> NUMA<=20: span=0-1 groups=(0,1)
>> NUMA<=22: span=0-2 groups=(0,1)->(0,2-3)
>> NUMA<=24: span=0-3 groups=(0-2)->(0,2-3)
>>
>> As you can see, the domain representing distance <= 20 will be degenerated
>> (it has a single group). If we were to e.g. add some more nodes to the left
>> of node0, then we would trigger the "grandchildren logic" for node1 and
>> would end up creating a reference to node1 NUMA<=20's sgc, which is a
>> mistake: that domain will be degenerated, and that sgc will never be
>> updated. The right thing to do here would be reference node1 NUMA<=12's
>> sgc, which the above snippet does.
>
> Guess I got your point even though the diagram is not correct :-)
>

Good!

> If the topology is as below(add a node left to node1 rather than
> node0):
>
>     9       2       10      2
> A <---> 1 <---> 0 <---> 2 <---> 3
>
> For nodeA,
> NUMA<=10: span=A   groups=(A)
> NUMA<=12: span= A groups= (A)
> NUMA<=19: span=A-1 groups=(A),(1)
> NUMA<=20: span=A-1 groups=(A,1)
> *1 NUMA<=21: span=A-1-0 groups=(A,1), node1's numa<=20
>
> For node0,
> NUMA<=10: span=9   groups=(0)
> #3 NUMA<=12: span=0-1 groups=(0)->(1)
> #2 NUMA<=19: span=0-1 groups=(0,1)
> #1 NUMA<=20: span=0-1-2 groups=(0,1),....
>
> *1 will firstly try #1, and it finds 2 is outside the A-1-0,
> then it will try #2. Finally #2 will be degenerated, so we
> should actually use #3. Amazing!
>

Bingo!

>>
>> >> +
>> >> +	return parent;
>> >> +}
>> >> +
>
> Thanks
> Barry