Re: [RFC PATCH] sched/topology: Introduce NUMA identity node sched domain

[Suravee Suthikulpanit]

On 8/11/17 16:15, Peter Zijlstra wrote:
> On Fri, Aug 11, 2017 at 12:58:22PM +0700, Suravee Suthikulpanit wrote:
> > On 8/11/17 11:57, Suravee Suthikulpanit wrote:
> > > > > [...]
> > > > > @@ -1445,9 +1448,24 @@ void sched_init_numa(void)
> > > > >          tl[i] = sched_domain_topology[i];
> > > > >
> > > > >      /*
> > > > > +     * Ignore the NUMA identity level if it has the same cpumask
> > > > > +     * as previous level. This is the case for:
> > > > > +     *   - System with last-level-cache (MC) sched domain span a NUMA node.
> > > > > +     *   - System with DIE sched domain span a NUMA node.
> > > > > +     *
> > > > > +     * Assume all NUMA nodes are identical, so only check node 0.
> > > > > +     */
> > > > > +    if (!cpumask_equal(sched_domains_numa_masks[0][0], tl[i-1].mask(0)))
> > > > > +        tl[i++] = (struct sched_domain_topology_level){
> > > > > +            .mask = sd_numa_mask,
> > > > > +            .numa_level = 0,
> > > > > +            SD_INIT_NAME(NODE)
> > > > > +        };
> > > >
> > > > So what you've forgotten to mention is that for those systems where the
> > > > LLC == NODE this now superfluous level gets removed by the degenerate
> > > > code. Have you verified that does the right thing?
> > >
> > > Let me check with that one and get back.
> >
> > Actually, it is not removed by the degenerate code. That is what this logic
> > is for. It checks for LCC == NODE or DIE == NODE before setting up the NODE
> > sched level. I can update the comment. This has also been tested on system
> > w/ LLC == NODE.
>
> Why does the degenerate code fail to remove things?

Sorry for confusion. Actually, the degenerate code does remove the duplicate 
NODE sched-domain.

The logic above is taking a different approach. Instead of depending on the 
degenerate code during cpu_attach_domain() at a later time, it would exclude the 
NODE sched-domain during sched_init_numa().  The difference is, without 
!cpumask_equal(), now the MC sched-domain would have the SD_PREFER_SIBLING flag 
set by the degenerate code since the flag got transferred down from the NODE to 
MC sched-domain. Would this be the preferred behavior for MC sched-domain?

Regards,
Suravee