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

["Chen, Yu C" <yu.c.chen@intel.com>]

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?

thanks,
Chenyu