fix(bpf): get_on_cpu_cycles not returning delta

Current code assigns val to c.counter, which is always
0 since it's not used by bpf_perf_event_read_value -
we're using bpf_perf_event_read in this function since the
former produces a strange verifier error.

In addition, add some logging around eBPF array resizing.

Fixes: #1402

Signed-off-by: Dave Tucker <[email protected]>

bpfassets/libbpf/src/kepler.bpf.c

@@ -126,19 +126,18 @@ static inline u64 calc_delta(u64 *prev_val, u64 *val)
 
 static inline u64 get_on_cpu_cycles(u32 *cpu_id)
 {
-	u64 delta, val, *prev_val;
-	struct bpf_perf_event_value c;
-	int error;
+	u64 delta, *prev_val, count;
+	s64 error;
 
 	// TODO: Fix Verifier errors upon changing this to bpf_perf_event_read_value
-	error = bpf_perf_event_read(&cpu_cycles_event_reader, *cpu_id);
-	if (error < 0) {
-		return delta;
-	}
-	val = c.counter;
+	count = bpf_perf_event_read(&cpu_cycles_event_reader, *cpu_id);
+	error = (s64)count;
+	if (error <= -2 && error >= -22)
+		return 0;
+
 	prev_val = bpf_map_lookup_elem(&cpu_cycles, cpu_id);
-	delta = calc_delta(prev_val, &val);
-	bpf_map_update_elem(&cpu_cycles, cpu_id, &val, BPF_ANY);
+	delta = calc_delta(prev_val, &count);
+	bpf_map_update_elem(&cpu_cycles, cpu_id, &count, BPF_ANY);
 
 	return delta;
 }

pkg/bpfassets/attacher/libbpf_attacher.go

@@ -110,6 +110,7 @@ func attachLibbpfModule() (*bpf.Module, error) {
 		return nil, fmt.Errorf("failed to load module: %v", err)
 	}
 	// resize array entries
+	klog.Infof("%d CPU cores detected. Resizing eBPF Perf Event Arrays", cpuCores)
 	for _, arrayName := range bpfArrays {
 		err = resizeArrayEntries(arrayName, cpuCores)
 		if err != nil {