* Peter Zijlstra <[email protected]> [2018-08-08 09:58:41]: > On Wed, Aug 08, 2018 at 12:39:31PM +0530, Srikar Dronamraju wrote: > > - numa topology on 2 node systems no more marked as NUMA_DIRECT. After > > this commit, it gets reported as NUMA_BACKPLANE. This is because > > sched_domains_numa_level now equals 2 on 2 node systems. > > > > - Extra numa sched domain that gets added and degenerated on most > > machines. The Identity node is only needed on very few systems. > > Also all non-numa systems will end up populating > > sched_domains_numa_distance and sched_domains_numa_masks tables. > > > > - On shared lpars like powerpc, this extra sched domain creation can > > lead to repeated rcu stalls, sometimes even causing unresponsive > > systems on boot. On such stalls, it was noticed that > > init_sched_groups_capacity() (sg != sd->groups is always true). > > The idea was that if the topology level is redundant (as it often is); > then the degenerate code would take it out. > > Why is that not working (right) and can we fix that instead? >
All I have found is regular NUMA sched_domains use OVERLAP flag, which inturn results in build_overlap_sched_groups(). The Identity node uses build_sched_groups. Somehow build_sched_groups is unable to create the group list. I still getting to see why that makes a difference. I have tried with passing .flags = SDTL_OVERLAP to the identity node and that works well. However, I still think if majority of the cases the identity node is going to be redundant, then we should use hint. We could fix the numa topology to be NUMA_DIRECT for 2 node machines, by checking if sched_domains_numa_levels == 2, but then I dont know what it means for a system that has only NODE but not NUMA level. i.e what should we say the numa topology type of a machine that has only NODE but not NUMA sched_domain? -- Thanks and Regards Srikar
