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.