select_rq_task() already checked that 'p->nr_cpus_allowed > 1',
'p->nr_cpus_allowed == 1' checker in scx_select_cpu_dfl() is redundant.

Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
---
 kernel/sched/ext.c | 11 +----------
 1 file changed, 1 insertion(+), 10 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index eabf8705e1e6..e91d5e9caa2f 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3051,22 +3051,13 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
 	 * there is an idle core elsewhere on the system.
 	 */
 	cpu = smp_processor_id();
-	if ((wake_flags & SCX_WAKE_SYNC) && p->nr_cpus_allowed > 1 &&
+	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))
 			goto cpu_found;
 	}
 
-	if (p->nr_cpus_allowed == 1) {
-		if (test_and_clear_cpu_idle(prev_cpu)) {
-			cpu = prev_cpu;
-			goto cpu_found;
-		} else {
-			return prev_cpu;
-		}
-	}
-
 	/*
 	 * If CPU has SMT, any wholly idle CPU is likely a better pick than
 	 * partially idle @prev_cpu.
-- 
2.33.0

On Thu, Sep 26, 2024 at 06:39:49PM +0800, Zhang Qiao wrote:
> select_rq_task() already checked that 'p->nr_cpus_allowed > 1',
> 'p->nr_cpus_allowed == 1' checker in scx_select_cpu_dfl() is redundant.
> 
> Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>

Applied to sched_ext/for-6.12-fixes. I think we probably need a flag to tell
the BPF scheduler whether select_cpu has been called or not. Right now, most
schedulers assume that select_cpu() as called before enqueue() on wakeup
path but that's not true for CPU-affine tasks.

Thanks.

-- 
tejun