[PATCH sched_ext/for-7.1] tools/sched_ext: Kick idle CPU for pinned tasks in scx_qmap

[Tejun Heo]

scx_qmap uses global BPF queue maps for task dispatch. A task pinned to a
single CPU can only be dispatched by its home CPU's ops.dispatch(), but an
idle CPU won't call ops.dispatch() on its own. This leaves per-CPU kthreads
like ksoftirqd stranded, causing NOHZ tick-stop warnings from pending
softirqs.

Kick the target CPU with SCX_KICK_IDLE when enqueueing a pinned task.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 tools/sched_ext/scx_qmap.bpf.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
index f3587fb709c9..09d1624fb869 100644
--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -314,6 +314,14 @@ void BPF_STRUCT_OPS(qmap_enqueue, struct task_struct *p, u64 enq_flags)
 		__sync_fetch_and_add(&nr_highpri_queued, 1);
 	}
 	__sync_fetch_and_add(&nr_enqueued, 1);
+
+	/*
+	 * Kick idle target CPU for pinned tasks. Without this, the CPU can
+	 * idle while ksoftirqd is pending in the BPF queue, triggering NOHZ
+	 * tick-stop warnings.
+	 */
+	if (p->nr_cpus_allowed == 1)
+		scx_bpf_kick_cpu(scx_bpf_task_cpu(p), SCX_KICK_IDLE);
 }

 /*
--
2.53.0