Re: [RESEND PATCH] sched/fair: Skip sched_balance_running cmpxchg when balance is not due

[Shrikanth Hegde]

On 10/13/25 10:02 PM, Chen, Yu C wrote:
> On 10/13/2025 10:26 PM, Peter Zijlstra wrote:
> > On Thu, Oct 02, 2025 at 04:00:12PM -0700, Tim Chen wrote:
> >
> > > During load balancing, balancing at the LLC level and above must be
> > > serialized.
> >
> > I would argue the wording here, there is no *must*, they *are*. Per the
> > current rules SD_NUMA and up get SD_SERIALIZE.
> >
> > This is a *very* old thing, done by Christoph Lameter back when he was
> > at SGI. I'm not sure this default is still valid or not. Someone would
> > have to investigate. An alternative would be moving it into
> > node_reclaim_distance or somesuch.
>
> Do you mean the following:
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 444bdfdab731..436c899d8da2 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1697,11 +1697,16 @@ sd_init(struct sched_domain_topology_level *tl,
>                  sd->cache_nice_tries = 2;
>
>                  sd->flags &= ~SD_PREFER_SIBLING;
> -               sd->flags |= SD_SERIALIZE;
>                  if (sched_domains_numa_distance[tl->numa_level] > 
> node_reclaim_distance) {
>                          sd->flags &= ~(SD_BALANCE_EXEC |
>                                         SD_BALANCE_FORK |
>                                         SD_WAKE_AFFINE);
> +                       /*
> +                        * Nodes that are far away need to be serialized to
> +                        * reduce the overhead of long-distance task 
> migration
> +                        * caused by load balancing.
> +                        */
> +                       sd->flags |= SD_SERIALIZE;
>                  }
>
> We can launch some tests to see if removing SD_SERIALIZE would
> bring any impact.
>
> > > On a 2-socket Granite Rapids system with sub-NUMA clustering enabled
> > > and running OLTP workloads, 7.6% of CPU cycles were spent on cmpxchg
> > > operations for `sched_balance_running`. In most cases, the attempt
> > > aborts immediately after acquisition because the load balance time is
> > > not yet due.
> >
> > So I'm not sure I understand the situation, @continue_balancing should
> > limit this concurrency to however many groups are on this domain -- your
> > granite thing with SNC on would have something like 6 groups?
>
> My understanding is that, continue_balancing is set to false after
> atomic_cmpxhg(sched_balance_running), so if sched_balance_domains()
> is invoked by many CPUs in parallel, the atomic operation still compete?

From what i could remember,

This mostly always happens at SMT after which continue_balancing = 0.
Since multiple CPUs end up calling it (specially on busy system)
it causes a lot cacheline bouncing. and ends up showing in perf profile.