delay-kfunc: Calibrate both inner and outer loop counts

To make it possible to hit the whole range of delays, we need to vary
the number of iterations in the inner loop as well, or we'll hit the
loop counter limit for the outer loop. So start with a lower number of
iterations for the inner loop, and increase it as part of the
calibration as needed.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

Author: tohojo

delay-kfunc/delay-kfunc.bpf.c

@@ -11,14 +11,15 @@
 char LICENSE[] SEC("license") = "GPL";
 
 #define TIME_ROUNDS 10
-#define TIME_ITER 100
+#define TIME_ITER 1000
 
 struct {
 	__uint(type, BPF_MAP_TYPE_RINGBUF);
 	__uint(max_entries, 0x1000);
 } delay_ringbuf SEC(".maps");
 
-__u64 loop_iterations = 10000;
+__u64 iterations_inner = 10000;
+__u64 iterations_outer = 10000;
 __u64 avg_delay = 100;
 
 static int recurse_loop(__u64 idx, void *ctx)
@@ -28,7 +29,7 @@ static int recurse_loop(__u64 idx, void *ctx)
 
 static int run_delay(__u64 idx, void *ctx)
 {
-	bpf_loop(100000, recurse_loop, NULL, 0);
+	bpf_loop(iterations_inner, recurse_loop, NULL, 0);
 	return 0;
 }
 
@@ -41,7 +42,7 @@ int BPF_PROG(delay_function)
 	int ret;
 
 	start_ns = bpf_ktime_get_boot_ns();
-	ret = bpf_loop(loop_iterations, run_delay, NULL, 0);
+	ret = bpf_loop(iterations_outer, run_delay, NULL, 0);
 
 	stats = bpf_ringbuf_reserve(&delay_ringbuf, sizeof(*stats), 0);
 	if (stats) {

delay-kfunc/delay-kfunc.c

@@ -106,9 +106,9 @@ static int process_stats_entry(void *ctx, void *data, size_t len)
 
 int main(int argc, char *argv[])
 {
+	unsigned long loop_outer, loop_inner;
 	struct ring_buffer *rb = NULL;
 	struct delay_kfunc_bpf *skel;
-	unsigned long loop_iter;
 	long target_delay;
 	char *func_name;
 	int err = -1;
@@ -131,6 +131,9 @@ int main(int argc, char *argv[])
 	if (!skel)
 		goto out;
 
+	loop_outer = skel->data->iterations_outer;
+	loop_inner = skel->data->iterations_inner;
+
 	err = bpf_program__set_attach_target(skel->progs.delay_function, 0,
 					     func_name);
 	if (err) {
@@ -149,12 +152,18 @@ int main(int argc, char *argv[])
 	if (err)
 		goto out;
 
-	loop_iter = 1 + target_delay * 1000 / skel->data->avg_delay;
-	if (loop_iter > 1 << 23)
-		loop_iter = 1 << 23;
-	printf("average outer loop delay %llu ns - looping %lu times to hit target\n",
-	       skel->data->avg_delay, loop_iter);
-	skel->data->loop_iterations = loop_iter;
+	printf("average loop duration %llu ns / %lu iterations ",
+	       skel->data->avg_delay,
+	       loop_inner);
+
+	loop_outer = 1 + target_delay * 1000 / skel->data->avg_delay;
+	while (loop_outer > 1 << 23) {
+		loop_inner *= 10;
+		loop_outer /= 10;
+	}
+	printf("- looping %lu * %lu times to hit target\n", loop_outer, loop_inner);
+	skel->data->iterations_outer = loop_outer;
+	skel->data->iterations_inner = loop_inner;
 
 	rb = ring_buffer__new(bpf_map__fd(skel->maps.delay_ringbuf),
 			      process_stats_entry,