Re: [PATCH] sched/topology: Use Identity node only if required

[Peter Zijlstra]

On Wed, Aug 08, 2018 at 12:39:31PM +0530, Srikar Dronamraju wrote:
> With Commit 051f3ca02e46 ("sched/topology: Introduce NUMA identity node
> sched domain") scheduler introduces an extra numa level. However that
> leads to
> 
>  - 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?