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

[Srikar Dronamraju]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> [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