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

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