Re: [PATCH v2] sched/topology: fix the issue groups don't span domain->span for NUMA diameter > 2

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Feb 04, 2021 at 12:12:01AM +1300, Barry Song wrote:
> As long as NUMA diameter > 2, building sched_domain by sibling's child
> domain will definitely create a sched_domain with sched_group which will
> span out of the sched_domain:
> 
>                +------+         +------+        +-------+       +------+
>                | node |  12     |node  | 20     | node  |  12   |node  |
>                |  0   +---------+1     +--------+ 2     +-------+3     |
>                +------+         +------+        +-------+       +------+
> 
> domain0        node0            node1            node2          node3
> 
> domain1        node0+1          node0+1          node2+3        node2+3
>                                                  +
> domain2        node0+1+2                         |
>              group: node0+1                      |
>                group:node2+3 <-------------------+
> 
> when node2 is added into the domain2 of node0, kernel is using the child
> domain of node2's domain2, which is domain1(node2+3). Node 3 is outside
> the span of the domain including node0+1+2.
> 
> This will make load_balance() run based on screwed avg_load and group_type
> in the sched_group spanning out of the sched_domain, and it also makes
> select_task_rq_fair() pick an idle CPU out of the sched_domain.
> 
> Real servers which suffer from this problem include Kunpeng920 and 8-node
> Sun Fire X4600-M2, at least.
> 
> Here we move to use the *child* domain of the *child* domain of node2's
> domain2 as the new added sched_group. At the same time, we re-use the
> lower level sgc directly.
> 
>                +------+         +------+        +-------+       +------+
>                | node |  12     |node  | 20     | node  |  12   |node  |
>                |  0   +---------+1     +--------+ 2     +-------+3     |
>                +------+         +------+        +-------+       +------+
> 
> domain0        node0            node1          +- node2          node3
>                                                |
> domain1        node0+1          node0+1        | node2+3        node2+3
>                                                |
> domain2        node0+1+2                       |
>              group: node0+1                    |
>                group:node2 <-------------------+
> 

I've finally had a moment to think about this, would it make sense to
also break up group: node0+1, such that we then end up with 3 groups of
equal size?

> w/ patch, we don't get "groups don't span domain->span" any more:
> [    1.486271] CPU0 attaching sched-domain(s):
> [    1.486820]  domain-0: span=0-1 level=MC
> [    1.500924]   groups: 0:{ span=0 cap=980 }, 1:{ span=1 cap=994 }
> [    1.515717]   domain-1: span=0-3 level=NUMA
> [    1.515903]    groups: 0:{ span=0-1 cap=1974 }, 2:{ span=2-3 cap=1989 }
> [    1.516989]    domain-2: span=0-5 level=NUMA
> [    1.517124]     groups: 0:{ span=0-3 cap=3963 }, 4:{ span=4-5 cap=1949 }

		     groups: 0:{ span=0-1 cap=1974 }, 2:{ span=2-3, cap=1989 }, 4:{ span=4-5, cap=1949 }

> [    1.517369]     domain-3: span=0-7 level=NUMA
> [    1.517423]      groups: 0:{ span=0-5 mask=0-1 cap=5912 }, 6:{ span=4-7 mask=6-7 cap=4054 }

Let me continue to think about this... it's been a while :/