[PATCH] sched_ext: improve WAKE_SYNC behavior for default idle CPU selection

[Andrea Righi <andrea.righi@linux.dev>]

In the sched_ext built-in idle CPU selection logic, when handling a
WF_SYNC wakeup, we always attempt to migrate the task to the waker's
CPU, as the waker is expected to yield the CPU after waking the task.

However, it may be preferable to keep the task on its previous CPU if
the waker's CPU is cache-affine.

The same approach is also used by the fair class and in other scx
schedulers, like scx_rusty and scx_bpfland.

Therefore, apply the same logic to the built-in idle CPU selection
policy as well.

Signed-off-by: Andrea Righi <andrea.righi@linux.dev>
---
 kernel/sched/ext.c | 44 +++++++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 6eae3b69bf6e..8b7d90db3cfb 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3087,20 +3087,38 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
 	*found = false;
 
 	/*
-	 * If WAKE_SYNC, the waker's local DSQ is empty, and the system is
-	 * under utilized, wake up @p to the local DSQ of the waker. Checking
-	 * only for an empty local DSQ is insufficient as it could give the
-	 * wakee an unfair advantage when the system is oversaturated.
-	 * Checking only for the presence of idle CPUs is also insufficient as
-	 * the local DSQ of the waker could have tasks piled up on it even if
-	 * there is an idle core elsewhere on the system.
-	 */
-	cpu = smp_processor_id();
-	if ((wake_flags & SCX_WAKE_SYNC) &&
-	    !cpumask_empty(idle_masks.cpu) && !(current->flags & PF_EXITING) &&
-	    cpu_rq(cpu)->scx.local_dsq.nr == 0) {
-		if (cpumask_test_cpu(cpu, p->cpus_ptr))
+	 * If WAKE_SYNC, try to migrate the wakee to the waker's CPU.
+	 */
+	if (wake_flags & SCX_WAKE_SYNC) {
+		cpu = smp_processor_id();
+
+		/*
+		 * If the waker's CPU is cache affine and prev_cpu is idle,
+		 * then avoid a migration.
+		 */
+		if (cpus_share_cache(cpu, prev_cpu) &&
+		    test_and_clear_cpu_idle(prev_cpu))
 			goto cpu_found;
+
+		/*
+		 * If the waker's local DSQ is empty, and the system is under
+		 * utilized, try to wake up @p to the local DSQ of the waker.
+		 *
+		 * Checking only for an empty local DSQ is insufficient as it
+		 * could give the wakee an unfair advantage when the system is
+		 * oversaturated.
+		 *
+		 * Checking only for the presence of idle CPUs is also
+		 * insufficient as the local DSQ of the waker could have tasks
+		 * piled up on it even if there is an idle core elsewhere on
+		 * the system.
+		 */
+		if (!cpumask_empty(idle_masks.cpu) &&
+		    !(current->flags & PF_EXITING) &&
+		    cpu_rq(cpu)->scx.local_dsq.nr == 0) {
+			if (cpumask_test_cpu(cpu, p->cpus_ptr))
+				goto cpu_found;
+		}
 	}
 
 	/*
-- 
2.47.0