Re: [PATCH bpf-next 2/4] libbpf: ringbuf: Add overwrite ring buffer process

[Zvi Effron <zeffron@riotgames.com>]

On Sun, Aug 3, 2025 at 7:27 PM Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>
> From: Xu Kuohai <xukuohai@huawei.com>
>
> In overwrite mode, the producer does not wait for the consumer, so the
> consumer is responsible for handling conflicts. An optimistic method
> is used to resolve the conflicts: the consumer first reads consumer_pos,
> producer_pos and overwrite_pos, then calculates a read window and copies
> data in the window from the ring buffer. After copying, it checks the
> positions to decide if the data in the copy window have been overwritten
> by be the producer. If so, it discards the copy and tries again. Once
> success, the consumer processes the events in the copy.
>
> Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
> ---
> tools/lib/bpf/ringbuf.c | 103 +++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 102 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
> index 9702b70da444..9c072af675ff 100644
> --- a/tools/lib/bpf/ringbuf.c
> +++ b/tools/lib/bpf/ringbuf.c
> @@ -27,10 +27,13 @@ struct ring {
> ring_buffer_sample_fn sample_cb;
> void *ctx;
> void *data;
> + void *read_buffer;
> unsigned long *consumer_pos;
> unsigned long *producer_pos;
> + unsigned long *overwrite_pos;
> unsigned long mask;
> int map_fd;
> + bool overwrite_mode;
> };
>
> struct ring_buffer {
> @@ -69,6 +72,9 @@ static void ringbuf_free_ring(struct ring_buffer *rb, struct ring *r)
> r->producer_pos = NULL;
> }
>
> + if (r->read_buffer)
> + free(r->read_buffer);
> +
> free(r);
> }
>
> @@ -119,6 +125,14 @@ int ring_buffer__add(struct ring_buffer *rb, int map_fd,
> r->sample_cb = sample_cb;
> r->ctx = ctx;
> r->mask = info.max_entries - 1;
> + r->overwrite_mode = info.map_flags & BPF_F_OVERWRITE;
> + if (unlikely(r->overwrite_mode)) {
> + r->read_buffer = malloc(info.max_entries);
> + if (!r->read_buffer) {
> + err = -ENOMEM;
> + goto err_out;
> + }
> + }
>
> /* Map writable consumer page */
> tmp = mmap(NULL, rb->page_size, PROT_READ | PROT_WRITE, MAP_SHARED, map_fd, 0);
> @@ -148,6 +162,7 @@ int ring_buffer__add(struct ring_buffer *rb, int map_fd,
> goto err_out;
> }
> r->producer_pos = tmp;
> + r->overwrite_pos = r->producer_pos + 1; /* overwrite_pos is next to producer_pos */
> r->data = tmp + rb->page_size;
>
> e = &rb->events[rb->ring_cnt];
> @@ -232,7 +247,7 @@ static inline int roundup_len(__u32 len)
> return (len + 7) / 8 * 8;
> }
>
> -static int64_t ringbuf_process_ring(struct ring *r, size_t n)
> +static int64_t ringbuf_process_normal_ring(struct ring *r, size_t n)
> {
> int *len_ptr, len, err;
> /* 64-bit to avoid overflow in case of extreme application behavior */
> @@ -278,6 +293,92 @@ static int64_t ringbuf_process_ring(struct ring *r, size_t n)
> return cnt;
> }
>
> +static int64_t ringbuf_process_overwrite_ring(struct ring *r, size_t n)
> +{
> +
> + int err;
> + uint32_t *len_ptr, len;
> + /* 64-bit to avoid overflow in case of extreme application behavior */
> + int64_t cnt = 0;
> + size_t size, offset;
> + unsigned long cons_pos, prod_pos, over_pos, tmp_pos;
> + bool got_new_data;
> + void *sample;
> + bool copied;
> +
> + size = r->mask + 1;
> +
> + cons_pos = smp_load_acquire(r->consumer_pos);
> + do {
> + got_new_data = false;
> +
> + /* grab a copy of data */
> + prod_pos = smp_load_acquire(r->producer_pos);
> + do {
> + over_pos = READ_ONCE(*r->overwrite_pos);
> + /* prod_pos may be outdated now */
> + if (over_pos < prod_pos) {
> + tmp_pos = max(cons_pos, over_pos);
> + /* smp_load_acquire(r->producer_pos) before
> + * READ_ONCE(*r->overwrite_pos) ensures that
> + * over_pos + r->mask < prod_pos never occurs,
> + * so size is never larger than r->mask
> + */
> + size = prod_pos - tmp_pos;
> + if (!size)
> + goto done;
> + memcpy(r->read_buffer,
> + r->data + (tmp_pos & r->mask), size);
> + copied = true;
> + } else {
> + copied = false;
> + }
> + prod_pos = smp_load_acquire(r->producer_pos);
> + /* retry if data is overwritten by producer */
> + } while (!copied || prod_pos - tmp_pos > r->mask);

This seems to allow for a situation where a call to process the ring can
infinite loop if the producers are producing and overwriting fast enough. That
seems suboptimal to me?

Should there be a timeout or maximum number of attempts or something that
returns -EBUSY or another error to the user?

> +
> + cons_pos = tmp_pos;
> +
> + for (offset = 0; offset < size; offset += roundup_len(len)) {
> + len_ptr = r->read_buffer + (offset & r->mask);
> + len = *len_ptr;
> +
> + if (len & BPF_RINGBUF_BUSY_BIT)
> + goto done;
> +
> + got_new_data = true;
> + cons_pos += roundup_len(len);
> +
> + if ((len & BPF_RINGBUF_DISCARD_BIT) == 0) {
> + sample = (void *)len_ptr + BPF_RINGBUF_HDR_SZ;
> + err = r->sample_cb(r->ctx, sample, len);
> + if (err < 0) {
> + /* update consumer pos and bail out */
> + smp_store_release(r->consumer_pos,
> + cons_pos);
> + return err;
> + }
> + cnt++;
> + }
> +
> + if (cnt >= n)
> + goto done;
> + }
> + } while (got_new_data);
> +
> +done:
> + smp_store_release(r->consumer_pos, cons_pos);
> + return cnt;
> +}
> +
> +static int64_t ringbuf_process_ring(struct ring *r, size_t n)
> +{
> + if (likely(!r->overwrite_mode))
> + return ringbuf_process_normal_ring(r, n);
> + else
> + return ringbuf_process_overwrite_ring(r, n);
> +}
> +
> /* Consume available ring buffer(s) data without event polling, up to n
> * records.
> *
> --
> 2.43.0
>
>