linux-kernel - [PATCH v2] sched/fair: Skip sched_balance_running cmpxchg when balance is not due

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Message-Id: <248b775fc9030989c829d4061f6f85ae33dabe45.1761682932.git.tim.c.chen@linux.intel.com>
Date: Tue, 28 Oct 2025 13:23:30 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Chen Yu <yu.c.chen@...el.com>,
	Doug Nelson <doug.nelson@...el.com>,
	Mohini Narkhede <mohini.narkhede@...el.com>,
	linux-kernel@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: [PATCH v2] sched/fair: Skip sched_balance_running cmpxchg when balance is not due

The NUMA sched domain sets the SD_SERIALIZE flag by default, allowing
only one NUMA load balancing operation to run system-wide at a time.

Currently, each MC group leader in a NUMA domain attempts to acquire
the global sched_balance_running flag via cmpxchg() before checking
whether load balancing is due or whether it is the designated leader for
that NUMA domain. On systems with a large number of cores, this causes
significant cache contention on the shared sched_balance_running flag.

This patch reduces unnecessary cmpxchg() operations by first checking
whether the balance interval has expired. If load balancing is not due,
the attempt to acquire sched_balance_running is skipped entirely.

On a 2-socket Granite Rapids system with sub-NUMA clustering enabled,
running an OLTP workload, 7.8% of total CPU cycles were previously spent
in sched_balance_domain() contending on sched_balance_running before
this change.

         : 104              static __always_inline int arch_atomic_cmpxchg(atomic_t *v, int old, int new)
         : 105              {
         : 106              return arch_cmpxchg(&v->counter, old, new);
    0.00 :   ffffffff81326e6c:       xor    %eax,%eax
    0.00 :   ffffffff81326e6e:       mov    $0x1,%ecx
    0.00 :   ffffffff81326e73:       lock cmpxchg %ecx,0x2394195(%rip)        # ffffffff836bb010 <sched_balance_running>
         : 110              sched_balance_domains():
         : 12234            if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
   99.39 :   ffffffff81326e7b:       test   %eax,%eax
    0.00 :   ffffffff81326e7d:       jne    ffffffff81326e99 <sched_balance_domains+0x209>
         : 12238            if (time_after_eq(jiffies, sd->last_balance + interval)) {
    0.00 :   ffffffff81326e7f:       mov    0x14e2b3a(%rip),%rax        # ffffffff828099c0 <jiffies_64>
    0.00 :   ffffffff81326e86:       sub    0x48(%r14),%rax
    0.00 :   ffffffff81326e8a:       cmp    %rdx,%rax

After applying this fix, sched_balance_domain() is gone from
the profile and there is a 8% throughput improvement.

v2:
1. Rearrange the patch to get rid of an indent level per Peter's
   suggestion.
2. Updated the data from new run by OLTP team.

link to v1: https://lore.kernel.org/lkml/e27d5dcb724fe46acc24ff44670bc4bb5be21d98.1759445926.git.tim.c.chen@linux.intel.com/

Reviewed-by: Chen Yu <yu.c.chen@...el.com>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
Reviewed-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
Tested-by: Mohini Narkhede <mohini.narkhede@...el.com>
Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 kernel/sched/fair.c | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 25970dbbb279..a10c95e11ea5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12226,6 +12226,8 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
 		}
 
 		interval = get_sd_balance_interval(sd, busy);
+		if (time_before(jiffies, sd->last_balance + interval))
+			goto out;
 
 		need_serialize = sd->flags & SD_SERIALIZE;
 		if (need_serialize) {
@@ -12233,19 +12235,18 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
 				goto out;
 		}
 
-		if (time_after_eq(jiffies, sd->last_balance + interval)) {
-			if (sched_balance_rq(cpu, rq, sd, idle, &continue_balancing)) {
-				/*
-				 * The LBF_DST_PINNED logic could have changed
-				 * env->dst_cpu, so we can't know our idle
-				 * state even if we migrated tasks. Update it.
-				 */
-				idle = idle_cpu(cpu);
-				busy = !idle && !sched_idle_cpu(cpu);
-			}
-			sd->last_balance = jiffies;
-			interval = get_sd_balance_interval(sd, busy);
+		if (sched_balance_rq(cpu, rq, sd, idle, &continue_balancing)) {
+			/*
+			 * The LBF_DST_PINNED logic could have changed
+			 * env->dst_cpu, so we can't know our idle
+			 * state even if we migrated tasks. Update it.
+			 */
+			idle = idle_cpu(cpu);
+			busy = !idle && !sched_idle_cpu(cpu);
 		}
+		sd->last_balance = jiffies;
+		interval = get_sd_balance_interval(sd, busy);
+
 		if (need_serialize)
 			atomic_set_release(&sched_balance_running, 0);
 out:
-- 
2.32.0