ringbuf: add ReadBuffer method

Add a ReadBuffer method to ringbuf.Reader, analogous to perf.Reader.

    name                                   old time/op    new time/op    delta
    Reader/normal_epoll_with_timeout_-1-4    8.58µs ± 3%    9.07µs ± 4%     ~     (p=0.057 n=4+4)

    name                                   old alloc/op   new alloc/op   delta
    Reader/normal_epoll_with_timeout_-1-4      384B ± 0%      288B ± 0%  -25.00%  (p=0.029 n=4+4)

    name                                   old allocs/op  new allocs/op  delta
    Reader/normal_epoll_with_timeout_-1-4      4.00 ± 0%      1.00 ± 0%  -75.00%  (p=0.029 n=4+4)

ringbuf/reader.go

@@ -20,6 +20,8 @@ var (
 	errBusy    = errors.New("sample not committed yet")
 )
 
+var ringbufHeaderSize = binary.Size(ringbufHeader{})
+
 // ringbufHeader from 'struct bpf_ringbuf_hdr' in kernel/bpf/ringbuf.c
 type ringbufHeader struct {
 	Len   uint32
@@ -42,16 +44,22 @@ type Record struct {
 	RawSample []byte
 }
 
-func readRecord(rd *ringbufEventRing) (r Record, err error) {
+// Read a record from an event ring.
+//
+// buf must be at least ringbufHeaderSize bytes long. sampleBuf may be nil.
+func readRecord(rd *ringbufEventRing, buf, sampleBuf []byte) (r Record, err error) {
 	rd.loadConsumer()
-	var header ringbufHeader
-	err = binary.Read(rd, internal.NativeEndian, &header)
-	if err == io.EOF {
+
+	buf = buf[:ringbufHeaderSize]
+	if _, err := io.ReadFull(rd, buf); err == io.EOF {
 		return Record{}, err
+	} else if err != nil {
+		return Record{}, fmt.Errorf("read event header: %w", err)
 	}
 
-	if err != nil {
-		return Record{}, fmt.Errorf("can't read event header: %w", err)
+	header := ringbufHeader{
+		internal.NativeEndian.Uint32(buf[0:4]),
+		internal.NativeEndian.Uint32(buf[4:8]),
 	}
 
 	if header.isBusy() {
@@ -76,10 +84,15 @@ func readRecord(rd *ringbufEventRing) (r Record, err error) {
 		return Record{}, fmt.Errorf("%w", errDiscard)
 	}
 
-	data := make([]byte, dataLenAligned)
+	var data []byte
+	if cap(sampleBuf) < int(dataLenAligned) {
+		data = make([]byte, dataLenAligned)
+	} else {
+		data = sampleBuf[:dataLenAligned]
+	}
 
 	if _, err := io.ReadFull(rd, data); err != nil {
-		return Record{}, fmt.Errorf("can't read sample: %w", err)
+		return Record{}, fmt.Errorf("read sample: %w", err)
 	}
 
 	rd.storeConsumer()
@@ -96,6 +109,7 @@ type Reader struct {
 	mu          sync.Mutex
 	ring        *ringbufEventRing
 	epollEvents []unix.EpollEvent
+	header      []byte
 }
 
 // NewReader creates a new BPF ringbuf reader.
@@ -129,6 +143,7 @@ func NewReader(ringbufMap *ebpf.Map) (*Reader, error) {
 		poller:      poller,
 		ring:        ring,
 		epollEvents: make([]unix.EpollEvent, 1),
+		header:      make([]byte, ringbufHeaderSize),
 	}, nil
 }
 
@@ -159,6 +174,15 @@ func (r *Reader) Close() error {
 //
 // Calling Close interrupts the function.
 func (r *Reader) Read() (Record, error) {
+	return r.ReadBuffer(nil)
+}
+
+// ReadBuffer is like Read except that it allows reusing buffers.
+//
+// buf is used as Record.RawSample if it is large enough to hold the sample data.
+// If buf is too small a new buffer will be allocated instead. It is valid to
+// pass nil, in which case ReadBuffer behaves like Read.
+func (r *Reader) ReadBuffer(buf []byte) (Record, error) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 
@@ -172,7 +196,7 @@ func (r *Reader) Read() (Record, error) {
 			return Record{}, err
 		}
 
-		record, err := readRecord(r.ring)
+		record, err := readRecord(r.ring, r.header, buf)
 		if errors.Is(err, errBusy) || errors.Is(err, errDiscard) {
 			continue
 		}

ringbuf/reader_test.go

@@ -239,17 +239,21 @@ func BenchmarkReader(b *testing.B) {
 			b.ResetTimer()
 			b.ReportAllocs()
 
+			var sampleBuf []byte
 			for i := 0; i < b.N; i++ {
 				ret, _, err := prog.Test(buf)
 				if err != nil {
 					b.Fatal(err)
 				} else if errno := syscall.Errno(-int32(ret)); errno != 0 {
 					b.Fatal("Expected 0 as return value, got", errno)
 				}
-				_, err = rd.Read()
+
+				record, err := rd.ReadBuffer(sampleBuf)
 				if err != nil {
 					b.Fatal("Can't read samples:", err)
 				}
+
+				sampleBuf = record.RawSample
 			}
 		})
 	}