[PATCH sched_ext/for-7.1] sched_ext: idle: Prioritize idle SMT sibling

[Andrea Righi <arighi@nvidia.com>]

In the default built-in idle CPU selection policy, when @prev_cpu is
busy and no fully idle core is available, try to place the task on its
SMT sibling if that sibling is idle, before searching any other idle CPU
in the same LLC.

Migration to the sibling is cheap and keeps the task on the same core,
preserving L1 cache and reducing wakeup latency.

On large SMT systems this appears to consistently boost throughput by
roughly 2-3% on CPU-bound workloads (running a number of tasks equal to
the number of SMT cores).

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/ext_idle.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index c7e4052626979..e0c57355b33b8 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -616,6 +616,18 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 		goto out_unlock;
 	}
 
+	/*
+	 * Use @prev_cpu's sibling if it's idle.
+	 */
+	if (sched_smt_active()) {
+		for_each_cpu_and(cpu, cpu_smt_mask(prev_cpu), allowed) {
+			if (cpu == prev_cpu)
+				continue;
+			if (scx_idle_test_and_clear_cpu(cpu))
+				goto out_unlock;
+		}
+	}
+
 	/*
 	 * Search for any idle CPU in the same LLC domain.
 	 */
-- 
2.53.0