[PATCH] sched_ext: Fix timer pinning and return value in scx_central

[Wanwu Li]

central_timerfn() re-arms the timer with a hardcoded
BPF_F_TIMER_CPU_PIN flag and ignores the return value, defeating
central_init()'s -EINVAL fallback for kernels without the flag
(<6.7): on such kernels the first tick kills the timer permanently
with no diagnostic.  Honor timer_pinned and check the return like
the init path does.

Fixes: 22a920209ab6 ("sched_ext: Implement tickless support")
Signed-off-by: Wanwu Li <liwanwu@xxxxxxxxxx>
---
diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c
index 64dd60b3e922..65dae9e45400 100644
--- a/tools/sched_ext/scx_central.bpf.c
+++ b/tools/sched_ext/scx_central.bpf.c
@@ -299,6 +299,7 @@ static int central_timerfn(void *map, int *key, struct bpf_timer *timer)
 	u64 now = scx_bpf_now();
 	u64 nr_to_kick = nr_queued;
 	s32 i, curr_cpu;
+	int ret;
 
 	curr_cpu = bpf_get_smp_processor_id();
 	if (timer_pinned && (curr_cpu != central_cpu)) {
@@ -332,7 +333,10 @@ static int central_timerfn(void *map, int *key, struct bpf_timer *timer)
 		scx_bpf_kick_cpu(cpu, SCX_KICK_PREEMPT);
 	}
 
-	bpf_timer_start(timer, TIMER_INTERVAL_NS, BPF_F_TIMER_CPU_PIN);
+	ret = bpf_timer_start(timer, TIMER_INTERVAL_NS,
+			      timer_pinned ? BPF_F_TIMER_CPU_PIN : 0);
+	if (ret)
+		scx_bpf_error("bpf_timer_start failed (%d)", ret);
 	__sync_fetch_and_add(&nr_timers, 1);
 	return 0;
 }