Re: [PATCH] sched: Skip useless sched_balance_running acquisition if load balance is not due

[K Prateek Nayak]

On 4/16/2025 3:17 PM, Vincent Guittot wrote:
> > Sorry, forgot to add.
> >
> > Do we really need newidle running all the way till NUMA? or if it runs till PKG is it enough?
> > the regular (idle) can take care for NUMA by serializing it?
> >
> > -               if (sd->flags & SD_BALANCE_NEWIDLE) {
> > +               if (sd->flags & SD_BALANCE_NEWIDLE && !(sd->flags & SD_SERIALIZE)) {
>
> Why not just clearing SD_BALANCE_NEWIDLE in your sched domain when you
> set SD_SERIALIZE

I've some questions around "sched_balance_running":

o Since this is a single flag across the entire system, it also implies
  CPUs cannon concurrently do load balancing across different NUMA
  domains which seems reasonable since a load balance at lower NUMA
  domain can potentially change the "nr_numa_running" and
  "nr_preferred_running" stats for the higher domain but if this is the
  case, a newidle balance at lower NUMA domain can interfere with a
  concurrent busy / newidle load balancing at higher NUMA domain.
  Is this expected? Should newidle balance be serialized too?

  (P.S. I copied over the serialize logic from sched_balance_domains()
   into sched_balance_newidle() and did not see any difference in my
   testing but perhaps there are benchmarks out there that care for
   this)

o If the intention of SD_SERIALIZE was to actually "serializes
  load-balancing passes over large domains (above the NODE topology
  level)" as the comment above "sched_balance_running" states, and
  this question is specific to x86 - when enabling SNC on Intel or
  NPS on AMD servers, the first NUMA domain is in fact as big as the
  NODE (now PKG domain) if not smaller. Is it okay to clear
  SD_SERIALIZE for these domains since they are small enough now?

--
Thanks and Regards,
Prateek