On Wed, Feb 24, 2021 at 04:09:44PM +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 outside 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, 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 <-------------------+
While the lower level sgc is re-used, this patch only changes the remote sched_groups for those sched_domains playing grandchild trick, therefore, sgc->next_update is still safe since it's only touched by CPUs that have the group span as local group. And sgc->imbalance is also safe because sd_parent remains the same in load_balance and LB only tries other CPUs from the local group. Moreover, since local groups are not touched, they are still getting roughly equal size in a TL. And should_we_balance() only matters with local groups, so the pull probability of those groups are still roughly equal.
Reported-by: Valentin Schneider valentin.schneider@arm.com Tested-by: Meelis Roos mroos@linux.ee Reviewed-by: Valentin Schneider valentin.schneider@arm.com Signed-off-by: Barry Song song.bao.hua@hisilicon.com
Thanks!