perf: do not add offline CPUs

Adding nil to rings and eventFds for offline CPUs can cause issues when
iterating over these elements.
Therefore skip adding to rings and eventFds if the CPU is offline at the
time of initalization.

Signed-off-by: Florian Lehner <[email protected]>

perf/reader.go

@@ -219,8 +219,6 @@ func NewReaderWithOptions(array *ebpf.Map, perCPUBuffer int, opts ReaderOptions)
 		event, ring, err := newPerfEventRing(i, perCPUBuffer, opts)
 		if errors.Is(err, unix.ENODEV) {
 			// The requested CPU is currently offline, skip it.
-			rings = append(rings, nil)
-			eventFds = append(eventFds, nil)
 			continue
 		}
 
@@ -288,14 +286,10 @@ func (pr *Reader) Close() error {
 	defer pr.pauseMu.Unlock()
 
 	for _, ring := range pr.rings {
-		if ring != nil {
-			ring.Close()
-		}
+		ring.Close()
 	}
 	for _, event := range pr.eventFds {
-		if event != nil {
-			event.Close()
-		}
+		event.Close()
 	}
 	pr.rings = nil
 	pr.eventFds = nil
@@ -386,11 +380,6 @@ func (pr *Reader) ReadInto(rec *Record) error {
 			// Waking up userspace is expensive, make the most of it by checking
 			// all rings.
 			for _, ring := range pr.rings {
-				// Skip rings that are not currently enabled.
-				if ring == nil {
-					continue
-				}
-
 				ring.loadHead()
 				pr.epollRings = append(pr.epollRings, ring)
 			}