[PATCH 1/2] libbpf: ringbuf: allow to consume up to a certain amount of items

[Andrea Righi]

In some cases, instead of always consuming all items from ring buffers
in a greedy way, we may want to consume up to a certain amount of items,
for example when we need to copy items from the BPF ring buffer to a
limited user buffer.

This change allows to set an upper limit to the amount of items consumed
from one or more ring buffers.

Link: https://lore.kernel.org/lkml/20240310154726.734289-1-andrea.righi@xxxxxxxxxxxxx/T
Signed-off-by: Andrea Righi <andrea.righi@xxxxxxxxxxxxx>
---
 tools/lib/bpf/ringbuf.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
index aacb64278a01..81df535040d1 100644
--- a/tools/lib/bpf/ringbuf.c
+++ b/tools/lib/bpf/ringbuf.c
@@ -231,7 +231,7 @@ static inline int roundup_len(__u32 len)
 	return (len + 7) / 8 * 8;
 }
 
-static int64_t ringbuf_process_ring(struct ring *r)
+static int64_t ringbuf_process_ring(struct ring *r, int64_t max_items)
 {
 	int *len_ptr, len, err;
 	/* 64-bit to avoid overflow in case of extreme application behavior */
@@ -264,7 +264,14 @@ static int64_t ringbuf_process_ring(struct ring *r)
 							  cons_pos);
 					return err;
 				}
-				cnt++;
+				if (++cnt >= max_items) {
+					/* update consumer pos and return the
+					 * total amount of items consumed.
+					 */
+					smp_store_release(r->consumer_pos,
+							  cons_pos);
+					goto done;
+				}
 			}
 
 			smp_store_release(r->consumer_pos, cons_pos);
@@ -281,19 +288,18 @@ static int64_t ringbuf_process_ring(struct ring *r)
  */
 int ring_buffer__consume(struct ring_buffer *rb)
 {
-	int64_t err, res = 0;
+	int64_t err, res = 0, max_items = INT_MAX;
 	int i;
 
 	for (i = 0; i < rb->ring_cnt; i++) {
 		struct ring *ring = rb->rings[i];
 
-		err = ringbuf_process_ring(ring);
+		err = ringbuf_process_ring(ring, max_items);
 		if (err < 0)
 			return libbpf_err(err);
 		res += err;
+		max_items -= err;
 	}
-	if (res > INT_MAX)
-		return INT_MAX;
 	return res;
 }
 
@@ -304,7 +310,7 @@ int ring_buffer__consume(struct ring_buffer *rb)
 int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms)
 {
 	int i, cnt;
-	int64_t err, res = 0;
+	int64_t err, res = 0, max_items = INT_MAX;
 
 	cnt = epoll_wait(rb->epoll_fd, rb->events, rb->ring_cnt, timeout_ms);
 	if (cnt < 0)
@@ -314,13 +320,12 @@ int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms)
 		__u32 ring_id = rb->events[i].data.fd;
 		struct ring *ring = rb->rings[ring_id];
 
-		err = ringbuf_process_ring(ring);
+		err = ringbuf_process_ring(ring, max_items);
 		if (err < 0)
 			return libbpf_err(err);
 		res += err;
+		max_items -= err;
 	}
-	if (res > INT_MAX)
-		return INT_MAX;
 	return res;
 }
 
@@ -375,11 +380,11 @@ int ring__consume(struct ring *r)
 {
 	int64_t res;
 
-	res = ringbuf_process_ring(r);
+	res = ringbuf_process_ring(r, INT_MAX);
 	if (res < 0)
 		return libbpf_err(res);
 
-	return res > INT_MAX ? INT_MAX : res;
+	return res;
 }
 
 static void user_ringbuf_unmap_ring(struct user_ring_buffer *rb)
-- 
2.43.0