libbpf: Expose API to consume one ring at a time

We already provide ring_buffer__epoll_fd to enable use of external
polling systems. However, the only API available to consume the ring
buffer is ring_buffer__consume, which always checks all rings. When
polling for many events, this can be wasteful.

Signed-off-by: Adam Sindelar <adam@wowsignal.io>

tools/lib/bpf/libbpf.h

@@ -1195,6 +1195,7 @@ LIBBPF_API int ring_buffer__add(struct ring_buffer *rb, int map_fd,
 				ring_buffer_sample_fn sample_cb, void *ctx);
 LIBBPF_API int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms);
 LIBBPF_API int ring_buffer__consume(struct ring_buffer *rb);
+LIBBPF_API int ring_buffer__consume_ring(struct ring_buffer *rb, uint32_t ring_id);
 LIBBPF_API int ring_buffer__epoll_fd(const struct ring_buffer *rb);
 
 struct user_ring_buffer_opts {

tools/lib/bpf/ringbuf.c

@@ -290,6 +290,21 @@ int ring_buffer__consume(struct ring_buffer *rb)
 	return res;
 }
 
+/* Consume available data from a single RINGBUF map identified by its ID.
+ * The ring ID is returned in epoll_data by epoll_wait when called with
+ * ring_buffer__epoll_fd.
+ */
+int ring_buffer__consume_ring(struct ring_buffer *rb, uint32_t ring_id)
+{
+	struct ring *ring;
+
+	if (ring_id >= rb->ring_cnt)
+		return libbpf_err(-EINVAL);
+
+	ring = &rb->rings[ring_id];
+	return ringbuf_process_ring(ring);
+}
+
 /* Poll for available data and consume records, if any are available.
  * Returns number of records consumed (or INT_MAX, whichever is less), or
  * negative number, if any of the registered callbacks returned error.