Merge pull request #5 from sched-ext/htejun

scx_central: Implement fallback for missing BPF_F_TIMER_CPU_PIN support

Author: htejun

scheds/kernel-examples/scx_central.bpf.c

@@ -60,6 +60,7 @@ const volatile s32 central_cpu;
 const volatile u32 nr_cpu_ids = 1;	/* !0 for veristat, set during init */
 const volatile u64 slice_ns = SCX_SLICE_DFL;
 
+bool timer_pinned = true;
 u64 nr_total, nr_locals, nr_queued, nr_lost_pids;
 u64 nr_timers, nr_dispatches, nr_mismatches, nr_retries;
 u64 nr_overflows;
@@ -255,7 +256,7 @@ static int central_timerfn(void *map, int *key, struct bpf_timer *timer)
 	s32 i, curr_cpu;
 
 	curr_cpu = bpf_get_smp_processor_id();
-	if (curr_cpu != central_cpu) {
+	if (timer_pinned && (curr_cpu != central_cpu)) {
 		scx_bpf_error("Central timer ran on CPU %d, not central CPU %d",
 			      curr_cpu, central_cpu);
 		return 0;
@@ -308,12 +309,28 @@ int BPF_STRUCT_OPS_SLEEPABLE(central_init)
 	if (!timer)
 		return -ESRCH;
 
-	if (bpf_get_smp_processor_id() != central_cpu)
+	if (bpf_get_smp_processor_id() != central_cpu) {
+		scx_bpf_error("init from non-central CPU");
 		return -EINVAL;
+	}
 
 	bpf_timer_init(timer, &central_timer, CLOCK_MONOTONIC);
 	bpf_timer_set_callback(timer, central_timerfn);
+
 	ret = bpf_timer_start(timer, TIMER_INTERVAL_NS, BPF_F_TIMER_CPU_PIN);
+	/*
+	 * BPF_F_TIMER_CPU_PIN is pretty new (>=6.7). If we're running in a
+	 * kernel which doesn't have it, bpf_timer_start() will return -EINVAL.
+	 * Retry without the PIN. This would be the perfect use case for
+	 * bpf_core_enum_value_exists() but the enum type doesn't have a name
+	 * and can't be used with bpf_core_enum_value_exists(). Oh well...
+	 */
+	if (ret == -EINVAL) {
+		timer_pinned = false;
+		ret = bpf_timer_start(timer, TIMER_INTERVAL_NS, 0);
+	}
+	if (ret)
+		scx_bpf_error("bpf_timer_start failed (%d)", ret);
 	return ret;
 }
 

scheds/kernel-examples/scx_central.c

@@ -98,6 +98,9 @@ int main(int argc, char **argv)
 	link = bpf_map__attach_struct_ops(skel->maps.central_ops);
 	SCX_BUG_ON(!link, "Failed to attach struct_ops");
 
+	if (!skel->data->timer_pinned)
+		printf("WARNING : BPF_F_TIMER_CPU_PIN not available, timer not pinned to central\n");
+
 	while (!exit_req && !uei_exited(&skel->bss->uei)) {
 		printf("[SEQ %llu]\n", seq++);
 		printf("total   :%10lu    local:%10lu   queued:%10lu  lost:%10lu\n",