[PATCH] sched/fair: Skip sched-domain traversal when wake affine is disabled

[Srikar Dronamraju <srikar@linux.ibm.com>]

In select_task_rq_fair(), TTWU wakeups for which wake_wide() returns
true, or where the waking CPU is not in the task's allowed CPU mask,
want_affine is set to false.

For these wakeups, the scheduler does not perform wake-affine placement.
The existing code already treats this as a fast-path by exiting the
sched-domain walk before locating an SD_BALANCE_WAKE domain, ultimately
falling through to select_idle_sibling().

However, select_task_rq_fair() still enters the sched-domain traversal
only to break out almost immediately. Avoid this unnecessary work by
jumping directly to the existing fast path when want_affine is false.

As part of this cleanup:

  - Move want_affine into the WF_TTWU block where it is used.
  - Skip the sched-domain traversal entirely when want_affine is false.
  - Remove the now redundant want_affine test from the
    SD_WAKE_AFFINE check.
  - Remove the corresponding !want_affine loop termination logic.

This preserves existing wakeup placement behavior while avoiding
unnecessary sched-domain iteration for TTWU wakeups that cannot make
use of wake-affine placement.

Signed-off-by: Srikar Dronamraju <srikar@linux.ibm.com>
---
 kernel/sched/fair.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3ebec186f982..58ff55e093cb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8832,7 +8832,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 	struct sched_domain *tmp, *sd = NULL;
 	int cpu = smp_processor_id();
 	int new_cpu = prev_cpu;
-	int want_affine = 0;
 	/* SD_flags and WF_flags share the first nibble */
 	int sd_flag = wake_flags & 0xF;
 
@@ -8841,6 +8840,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 	 */
 	lockdep_assert_held(&p->pi_lock);
 	if (wake_flags & WF_TTWU) {
+		int want_affine;
+
 		record_wakee(p);
 
 		if ((wake_flags & WF_CURRENT_CPU) &&
@@ -8855,6 +8856,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 		}
 
 		want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, p->cpus_ptr);
+		if (!want_affine)
+			goto fast_path;
 	}
 
 	for_each_domain(cpu, tmp) {
@@ -8862,7 +8865,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 		 * If both 'cpu' and 'prev_cpu' are part of this domain,
 		 * cpu is a valid SD_WAKE_AFFINE target.
 		 */
-		if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
+		if ((tmp->flags & SD_WAKE_AFFINE) &&
 		    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
 			if (cpu != prev_cpu)
 				new_cpu = wake_affine(tmp, p, cpu, prev_cpu, sync);
@@ -8878,8 +8881,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 		 */
 		if (tmp->flags & sd_flag)
 			sd = tmp;
-		else if (!want_affine)
-			break;
 	}
 
 	/* Slow path */
@@ -8887,6 +8888,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 		return sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag);
 
 	/* Fast path */
+fast_path:
 	if (wake_flags & WF_TTWU)
 		return select_idle_sibling(p, prev_cpu, new_cpu);
 
-- 
2.43.0