[PATCH 6.18 039/175] sched/fair: Skip sched_balance_running cmpxchg when balance is not due

Greg Kroah-Hartman
2026-02-09T14:27:15Z

From: Tim Chen <tim.c.chen@linux.intel.com>

commit 3324b2180c17b21c31c16966cc85ca41a7c93703 upstream.

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 sched group leader directly under 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
load balancer 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
that the balancer is the designated leader for a NUMA domain from
should_we_balance(), and the balance interval has expired before
trying to acquire sched_balance_running to load balance a NUMA
domain.

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 5% throughput improvement.

[peterz: made it so that redo retains the 'lock' and split out the
CPU_NEWLY_IDLE change to a separate patch]
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.ibm.com>
Tested-by: Mohini Narkhede <mohini.narkhede@intel.com>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://patch.msgid.link/6fed119b723c71552943bfe5798c93851b30a361.1762800251.git.tim.c.chen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/sched/fair.c | 54 ++++++++++++++++++++++++++--------------------------
1 file changed, 28 insertions(+), 26 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11693,6 +11693,21 @@ static void update_lb_imbalance_stat(str
}

/*
+ * This flag serializes load-balancing passes over large domains
+ * (above the NODE topology level) - only one load-balancing instance
+ * may run at a time, to reduce overhead on very large systems with
+ * lots of CPUs and large NUMA distances.
+ *
+ * - Note that load-balancing passes triggered while another one
+ * is executing are skipped and not re-tried.
+ *
+ * - Also note that this does not serialize rebalance_domains()
+ * execution, as non-SD_SERIALIZE domains will still be
+ * load-balanced in parallel.
+ */
+static atomic_t sched_balance_running = ATOMIC_INIT(0);
+
+/*
* Check this_cpu to ensure it is balanced within domain. Attempt to move
* tasks if there is an imbalance.
*/
@@ -11717,6 +11732,7 @@ static int sched_balance_rq(int this_cpu
.fbq_type = all,
.tasks = LIST_HEAD_INIT(env.tasks),
};
+ bool need_unlock = false;

cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);

@@ -11728,6 +11744,14 @@ redo:
goto out_balanced;
}

+ if (!need_unlock && (sd->flags & SD_SERIALIZE) && idle != CPU_NEWLY_IDLE) {
+ int zero = 0;
+ if (!atomic_try_cmpxchg_acquire(&sched_balance_running, &zero, 1))
+ goto out_balanced;
+
+ need_unlock = true;
+ }
+
group = sched_balance_find_src_group(&env);
if (!group) {
schedstat_inc(sd->lb_nobusyg[idle]);
@@ -11968,6 +11992,9 @@ out_one_pinned:
sd->balance_interval < sd->max_interval)
sd->balance_interval *= 2;
out:
+ if (need_unlock)
+ atomic_set_release(&sched_balance_running, 0);
+
return ld_moved;
}

@@ -12093,21 +12120,6 @@ out_unlock:
}

/*
- * This flag serializes load-balancing passes over large domains
- * (above the NODE topology level) - only one load-balancing instance
- * may run at a time, to reduce overhead on very large systems with
- * lots of CPUs and large NUMA distances.
- *
- * - Note that load-balancing passes triggered while another one
- * is executing are skipped and not re-tried.
- *
- * - Also note that this does not serialize rebalance_domains()
- * execution, as non-SD_SERIALIZE domains will still be
- * load-balanced in parallel.
- */
-static atomic_t sched_balance_running = ATOMIC_INIT(0);
-
-/*
* Scale the max sched_balance_rq interval with the number of CPUs in the system.
* This trades load-balance latency on larger machines for less cross talk.
*/
@@ -12175,7 +12187,7 @@ static void sched_balance_domains(struct
/* Earliest time when we have to do rebalance again */
unsigned long next_balance = jiffies + 60*HZ;
int update_next_balance = 0;
- int need_serialize, need_decay = 0;
+ int need_decay = 0;
u64 max_cost = 0;

rcu_read_lock();
@@ -12199,13 +12211,6 @@ static void sched_balance_domains(struct
}

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)) {
if (sched_balance_rq(cpu, rq, sd, idle, &continue_balancing)) {
/*
@@ -12219,9 +12224,6 @@ static void sched_balance_domains(struct
sd->last_balance = jiffies;
interval = get_sd_balance_interval(sd, busy);
}
- 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;
update_next_balance = 1;