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Message-Id: <e27d5dcb724fe46acc24ff44670bc4bb5be21d98.1759445926.git.tim.c.chen@linux.intel.com>
Date: Thu, 2 Oct 2025 16:00:12 -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: [RESEND PATCH] sched/fair: Skip sched_balance_running cmpxchg when balance is not due
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@linux.intel.com/
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@...el.com>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
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
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