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

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Date:   Tue,  6 Sep 2022 21:56:44 +0200
From:   Francis Laniel <flaniel@...ux.microsoft.com>
To:     bpf@...r.kernel.org
Cc:     Francis Laniel <flaniel@...ux.microsoft.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        Joanne Koong <joannelkoong@...il.com>,
        Dave Marchevsky <davemarchevsky@...com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Maxim Mikityanskiy <maximmi@...dia.com>,
        Geliang Tang <geliang.tang@...e.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: [RFC PATCH v2 3/5] docs/bpf: Add documentation for overwritable ring buffer.

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

Signed-off-by: Francis Laniel <flaniel@...ux.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