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Per the > current rules SD_NUMA and up get SD_SERIALIZE. > > This is a *very* old thing, done by Christoph Lameter back when he was > at SGI. I'm not sure this default is still valid or not. Someone would > have to investigate. An alternative would be moving it into > node_reclaim_distance or somesuch. > Do you mean the following: diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c index 444bdfdab731..436c899d8da2 100644 --- a/kernel/sched/topology.c +++ b/kernel/sched/topology.c @@ -1697,11 +1697,16 @@ sd_init(struct sched_domain_topology_level *tl, sd->cache_nice_tries = 2; sd->flags &= ~SD_PREFER_SIBLING; - sd->flags |= SD_SERIALIZE; if (sched_domains_numa_distance[tl->numa_level] > node_reclaim_distance) { sd->flags &= ~(SD_BALANCE_EXEC | SD_BALANCE_FORK | SD_WAKE_AFFINE); + /* + * Nodes that are far away need to be serialized to + * reduce the overhead of long-distance task migration + * caused by load balancing. + */ + sd->flags |= SD_SERIALIZE; } We can launch some tests to see if removing SD_SERIALIZE would bring any impact. >> On a 2-socket Granite Rapids system with sub-NUMA clustering enabled >> and running OLTP workloads, 7.6% of CPU cycles were spent on cmpxchg >> operations for `sched_balance_running`. In most cases, the attempt >> aborts immediately after acquisition because the load balance time is >> not yet due. > > So I'm not sure I understand the situation, @continue_balancing should > limit this concurrency to however many groups are on this domain -- your > granite thing with SNC on would have something like 6 groups? > My understanding is that, continue_balancing is set to false after atomic_cmpxhg(sched_balance_running), so if sched_balance_domains() is invoked by many CPUs in parallel, the atomic operation still compete? thanks, Chenyu