Subject: Re: [PATCH v2 3/4] powerpc/numa: Early request for home node associativity

From: Srikar Dronamraju <[email-masked]>
Date: Fri, 6 Sep 2019 19:19:59 +0530

> > Regardless, I have an annoying question :-) Isn't it possible that,
> > while Linux is calling vphn_get_nid() for each logical cpu in sequence,
> > the platform could change a virtual processor's node assignment,
> > potentially causing sibling threads to get different node assignments
> > and producing an incoherent topology (which then leads to sched domain
> > assertions etc)?
> > 
> 
> Right, its certainly possible for node assignment to change while we iterate
> through the siblings. Do you have an recommendations?
> 

One thingk that I forgot to add was we already cache the cpu_to_node
mapping. If the mapping is around, we still don't lookup the nid.

However its still possible that in the first iteration where we cache the
nid's. we still end up with different nids for different siblings if node
associativity changes in between.

> > If so, I think more care is needed. The algorithm should make the vphn
> > call only once per cpu node, I think?
> 
> I didn't get "once per cpu node", How do we know which all cpus are part of
> that cpu node? Or did you mean once per cpu core?

-- 
Thanks and Regards
Srikar Dronamraju