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

[Tim Chen]

Repost comments:

There have been past discussions about avoiding serialization in load
balancing, but no objections were raised to this patch itself during
its last posting:
https://lore.kernel.org/lkml/20250416035823.1846307-1-tim.c.chen@xxxxxxxxxxxxxxx/

Vincent and Chen Yu have already provided their Reviewed-by tags.

We recently encountered this issue again on a 2-socket, 240-core
Clearwater Forest server running SPECjbb. In this case, 14% of CPU
cycles were wasted on unnecessary acquisitions of
sched_balance_running. This reinforces the need for the change, and we
hope it can be merged.

Tim

---

During load balancing, balancing at the LLC level and above must be
serialized. The scheduler currently checks the atomic
`sched_balance_running` flag before verifying whether a balance is
actually due. This causes high contention, as multiple CPUs may attempt
to acquire the flag concurrently.

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.

Fix this by checking whether a balance is due *before* trying to
acquire `sched_balance_running`. This avoids many wasted acquisitions
and reduces the cmpxchg overhead in `sched_balance_domain()` from 7.6%
to 0.05%. As a result, OLTP throughput improves by 11%.

Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8ce56a8d507f..bedd785c4a39 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12126,13 +12126,13 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
 
 		interval = get_sd_balance_interval(sd, busy);
 
-		need_serialize = sd->flags & SD_SERIALIZE;
-		if (need_serialize) {
-			if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
-				goto out;
-		}
-
 		if (time_after_eq(jiffies, sd->last_balance + interval)) {
+			need_serialize = sd->flags & SD_SERIALIZE;
+			if (need_serialize) {
+				if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
+					goto out;
+			}
+
 			if (sched_balance_rq(cpu, rq, sd, idle, &continue_balancing)) {
 				/*
 				 * The LBF_DST_PINNED logic could have changed
@@ -12144,9 +12144,9 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
 			}
 			sd->last_balance = jiffies;
 			interval = get_sd_balance_interval(sd, busy);
+			if (need_serialize)
+				atomic_set_release(&sched_balance_running, 0);
 		}
-		if (need_serialize)
-			atomic_set_release(&sched_balance_running, 0);
 out:
 		if (time_after(next_balance, sd->last_balance + interval)) {
 			next_balance = sd->last_balance + interval;
-- 
2.32.0