[tip: sched/core] sched/core: Get this cpu once in ttwu_queue_cond()

[tip-bot2 for Shrikanth Hegde]

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

Commit-ID:     0e81fe79fec5a639700f09f39c8ab680c3312ba2
Gitweb:        https://git.kernel.org/tip/0e81fe79fec5a639700f09f39c8ab680c3312ba2
Author:        Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
AuthorDate:    Tue, 24 Mar 2026 01:06:28 +05:30
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 24 Mar 2026 10:07:05 +01:00

sched/core: Get this cpu once in ttwu_queue_cond()

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 could add up to a small value.

ttwu_queue_cond is called with interrupt disabled. So preemption is
disabled. Hence cache the value once instead.

Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@xxxxxxxxx>
Link: https://patch.msgid.link/20260323193630.640311-3-sshegde@xxxxxxxxxxxxx
---
 kernel/sched/core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 64b467c..7c7d4bf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3842,6 +3842,8 @@ bool cpus_share_resources(int this_cpu, int that_cpu)
 
 static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
 {
+	int this_cpu = smp_processor_id();
+
 	/* See SCX_OPS_ALLOW_QUEUED_WAKEUP. */
 	if (!scx_allow_ttwu_queue(p))
 		return false;
@@ -3866,10 +3868,10 @@ static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
 	 * If the CPU does not share cache, then queue the task on the
 	 * remote rqs wakelist to avoid accessing remote data.
 	 */
-	if (!cpus_share_cache(smp_processor_id(), cpu))
+	if (!cpus_share_cache(this_cpu, cpu))
 		return true;
 
-	if (cpu == smp_processor_id())
+	if (cpu == this_cpu)
 		return false;
 
 	/*