linux-kernel - [PATCH] sched/fair: Save cpu id locally to avoid repeated smp_processor

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Message-ID: <20250814140141.2655087-1-zhongqiu.han@oss.qualcomm.com>
Date: Thu, 14 Aug 2025 22:01:41 +0800
From: Zhongqiu Han <zhongqiu.han@....qualcomm.com>
To: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com
Cc: linux-kernel@...r.kernel.org, quic_zhonhan@...cinc.com,
        zhongqiu.han@....qualcomm.com
Subject: [PATCH] sched/fair: Save cpu id locally to avoid repeated smp_processor_id() calls

Avoid repeated smp_processor_id() by saving cpu id in a local variable.

- find_new_ilb(): func called with interrupts disabled.
- sched_cfs_period_timer(): cpu id saved after raw_spin_lock_irqsave().

This improves clarity and reduces overhead without changing functionality.

Signed-off-by: Zhongqiu Han <zhongqiu.han@....qualcomm.com>
---
 kernel/sched/fair.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e256793b9a08..60a9830fb8a4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6401,6 +6401,8 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 	int count = 0;
 
 	raw_spin_lock_irqsave(&cfs_b->lock, flags);
+	int cpu = smp_processor_id();
+
 	for (;;) {
 		overrun = hrtimer_forward_now(timer, cfs_b->period);
 		if (!overrun)
@@ -6424,13 +6426,13 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 
 				pr_warn_ratelimited(
 	"cfs_period_timer[cpu%d]: period too short, scaling up (new cfs_period_us = %lld, cfs_quota_us = %lld)\n",
-					smp_processor_id(),
+					cpu,
 					div_u64(new, NSEC_PER_USEC),
 					div_u64(cfs_b->quota, NSEC_PER_USEC));
 			} else {
 				pr_warn_ratelimited(
 	"cfs_period_timer[cpu%d]: period too short, but cannot scale up without losing precision (cfs_period_us = %lld, cfs_quota_us = %lld)\n",
-					smp_processor_id(),
+					cpu,
 					div_u64(old, NSEC_PER_USEC),
 					div_u64(cfs_b->quota, NSEC_PER_USEC));
 			}
@@ -12195,12 +12197,13 @@ static inline int find_new_ilb(void)
 {
 	const struct cpumask *hk_mask;
 	int ilb_cpu;
+	int this_cpu = smp_processor_id();
 
 	hk_mask = housekeeping_cpumask(HK_TYPE_KERNEL_NOISE);
 
 	for_each_cpu_and(ilb_cpu, nohz.idle_cpus_mask, hk_mask) {
 
-		if (ilb_cpu == smp_processor_id())
+		if (ilb_cpu == this_cpu)
 			continue;
 
 		if (idle_cpu(ilb_cpu))
-- 
2.43.0