Calling smp_processor_id() on:
- In CONFIG_DEBUG_PREEMPT=y, if preemption/irq is disabled, then it does
not print any warning.
- In CONFIG_DEBUG_PREEMPT=n, it doesn't do anything apart from getting
__smp_processor_id

So with both CONFIG_DEBUG_PREEMPT=y/n, in preemption disabled section
it is better to cache the value. It could save a few cycles. Though
tiny, repeated in loop could add up to a small value.

find_new_ilb is called in interrupt context. So preemption is disabled.
So Hoist the this_cpu out of loop

Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
Split it from v1 and found a few more places. Rest of the bits of v1 is
worked out separate
v1: https://lore.kernel.org/all/20260319065314.343932-1-sshegde@linux.ibm.com/

 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1b62cb68faae..9298f49f842c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12626,14 +12626,14 @@ static inline int on_null_domain(struct rq *rq)
  */
 static inline int find_new_ilb(void)
 {
+	int this_cpu = smp_processor_id();
 	const struct cpumask *hk_mask;
 	int ilb_cpu;
 
 	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))
-- 
2.47.3

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     76504bce4ee6b8757647e07bc1710dcac9acdc2e
Gitweb:        https://git.kernel.org/tip/76504bce4ee6b8757647e07bc1710dcac9acdc2e
Author:        Shrikanth Hegde <sshegde@linux.ibm.com>
AuthorDate:    Tue, 24 Mar 2026 01:06:27 +05:30
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 24 Mar 2026 10:07:04 +01:00

sched/fair: Get this cpu once in find_new_ilb()

Calling smp_processor_id() on:
 - In CONFIG_DEBUG_PREEMPT=y, if preemption/irq is disabled, then it does
   not print any warning.
 - In CONFIG_DEBUG_PREEMPT=n, it doesn't do anything apart from getting
   __smp_processor_id

So with both CONFIG_DEBUG_PREEMPT=y/n, in preemption disabled section
it is better to cache the value. It could save a few cycles. Though
tiny, repeated in loop could add up to a small value.

find_new_ilb is called in interrupt context. So preemption is disabled.
So Hoist the this_cpu out of loop

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://patch.msgid.link/20260323193630.640311-2-sshegde@linux.ibm.com
---
 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0a35a82..2265092 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12614,14 +12614,14 @@ static inline int on_null_domain(struct rq *rq)
  */
 static inline int find_new_ilb(void)
 {
+	int this_cpu = smp_processor_id();
 	const struct cpumask *hk_mask;
 	int ilb_cpu;
 
 	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))