[RFC PATCH v2 3/5] docs/bpf: Add documentation for overwritable ring buffer.

Francis Laniel
2022-09-06T20:05:25Z

Add documentation to precise behavior of overwritable BPF ring buffer compared
to conventionnal ones.

Signed-off-by: Francis Laniel <flaniel@linux.microsoft.com>
---
Documentation/bpf/ringbuf.rst | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/Documentation/bpf/ringbuf.rst b/Documentation/bpf/ringbuf.rst
index 6a615cd62bda..e062381ff604 100644
--- a/Documentation/bpf/ringbuf.rst
+++ b/Documentation/bpf/ringbuf.rst
@@ -124,7 +124,7 @@ buffer. Currently 4 are supported:

- ``BPF_RB_AVAIL_DATA`` returns amount of unconsumed data in ring buffer;
- ``BPF_RB_RING_SIZE`` returns the size of ring buffer;
-- ``BPF_RB_CONS_POS``/``BPF_RB_PROD_POS`` returns current logical possition
+- ``BPF_RB_CONS_POS``/``BPF_RB_PROD_POS`` returns current logical position
of consumer/producer, respectively.

Returned values are momentarily snapshots of ring buffer state and could be
@@ -204,3 +204,19 @@ buffer. For extreme cases, when BPF program wants more manual control of
notifications, commit/discard/output helpers accept ``BPF_RB_NO_WAKEUP`` and
``BPF_RB_FORCE_WAKEUP`` flags, which give full control over notifications of
data availability, but require extra caution and diligence in using this API.
+
+Specific case of overwritable ring buffer
+-----------------------------------------
+
+Using ``BFP_F_RB_OVERWRITABLE`` when creating the ring buffer will make it
+overwritable.
+As a consequence, the producers will never be stopped from writing data, *i.e.*
+in this mode ``bpf_ringbuf_reserve()`` never blocks and returns NULL, but oldest
+events will be replaced by newest ones.
+
+In terms of implementation, this feature uses the same logic than overwritable
+perf ring buffer.
+The ring buffer is written backward, while it should be read forward from the
+producer position.
+As a consequence, in this mode, the consumer position has no meaning and can be
+used freely by userspace implementation.
--
2.25.1