[PATCH] sched/core: avoid calling select_task_rq if bound to one CPU for exec

[Jianyong Wu]

In the current implementation, even if the task calling execl is bound
to a single CPU (or not allowed to be migrated), it still invokes the
select_task_rq() callback to select a CPU. This is unnecessary and
wastes cycles.

Add a check: if the task is restricted to running on exactly one CPU
(cpus_ptr has only one online CPU) or is not allowed to be migrated,
skip select_task_rq() and directly use the task's bound CPU.

Test environment: 256-CPU X86 server
Test method: Run unixbench's execl test with task bound to a single CPU:

  $ numactl -C 10 ./Run execl -c 1

Test results: Average of 5 runs

baseline    patched    improvement
430.38      437.52     +1.66%

Co-developed-by: Yibin Liu <liuyibin@xxxxxxxx>
Signed-off-by: Yibin Liu <liuyibin@xxxxxxxx>
Signed-off-by: Jianyong Wu <wujianyong@xxxxxxxx>
---
 kernel/sched/core.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f754a60de848..c1e9f633cfb0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5442,7 +5442,11 @@ void sched_exec(void)
 	int dest_cpu;
 
 	scoped_guard (raw_spinlock_irqsave, &p->pi_lock) {
-		dest_cpu = p->sched_class->select_task_rq(p, task_cpu(p), WF_EXEC);
+		if (p->nr_cpus_allowed > 1 && !is_migration_disabled(p))
+			dest_cpu = p->sched_class->select_task_rq(p, task_cpu(p), WF_EXEC);
+		else
+			dest_cpu = cpumask_any(p->cpus_ptr);
+
 		if (dest_cpu == smp_processor_id())
 			return;
 
-- 
2.43.0