1===============
2BPF ring buffer
3===============
4
5This document describes BPF ring buffer design, API, and implementation details.
6
7.. contents::
8    :local:
9    :depth: 2
10
11Motivation
12----------
13
14There are two distinctive motivators for this work, which are not satisfied by
15existing perf buffer, which prompted creation of a new ring buffer
16implementation.
17
18- more efficient memory utilization by sharing ring buffer across CPUs;
19- preserving ordering of events that happen sequentially in time, even across
20  multiple CPUs (e.g., fork/exec/exit events for a task).
21
22These two problems are independent, but perf buffer fails to satisfy both.
23Both are a result of a choice to have per-CPU perf ring buffer.  Both can be
24also solved by having an MPSC implementation of ring buffer. The ordering
25problem could technically be solved for perf buffer with some in-kernel
26counting, but given the first one requires an MPSC buffer, the same solution
27would solve the second problem automatically.
28
29Semantics and APIs
30------------------
31
32Single ring buffer is presented to BPF programs as an instance of BPF map of
33type ``BPF_MAP_TYPE_RINGBUF``. Two other alternatives considered, but
34ultimately rejected.
35
36One way would be to, similar to ``BPF_MAP_TYPE_PERF_EVENT_ARRAY``, make
37``BPF_MAP_TYPE_RINGBUF`` could represent an array of ring buffers, but not
38enforce "same CPU only" rule. This would be more familiar interface compatible
39with existing perf buffer use in BPF, but would fail if application needed more
40advanced logic to lookup ring buffer by arbitrary key.
41``BPF_MAP_TYPE_HASH_OF_MAPS`` addresses this with current approach.
42Additionally, given the performance of BPF ringbuf, many use cases would just
43opt into a simple single ring buffer shared among all CPUs, for which current
44approach would be an overkill.
45
46Another approach could introduce a new concept, alongside BPF map, to represent
47generic "container" object, which doesn't necessarily have key/value interface
48with lookup/update/delete operations. This approach would add a lot of extra
49infrastructure that has to be built for observability and verifier support. It
50would also add another concept that BPF developers would have to familiarize
51themselves with, new syntax in libbpf, etc. But then would really provide no
52additional benefits over the approach of using a map.  ``BPF_MAP_TYPE_RINGBUF``
53doesn't support lookup/update/delete operations, but so doesn't few other map
54types (e.g., queue and stack; array doesn't support delete, etc).
55
56The approach chosen has an advantage of re-using existing BPF map
57infrastructure (introspection APIs in kernel, libbpf support, etc), being
58familiar concept (no need to teach users a new type of object in BPF program),
59and utilizing existing tooling (bpftool). For common scenario of using a single
60ring buffer for all CPUs, it's as simple and straightforward, as would be with
61a dedicated "container" object. On the other hand, by being a map, it can be
62combined with ``ARRAY_OF_MAPS`` and ``HASH_OF_MAPS`` map-in-maps to implement
63a wide variety of topologies, from one ring buffer for each CPU (e.g., as
64a replacement for perf buffer use cases), to a complicated application
65hashing/sharding of ring buffers (e.g., having a small pool of ring buffers
66with hashed task's tgid being a look up key to preserve order, but reduce
67contention).
68
69Key and value sizes are enforced to be zero. ``max_entries`` is used to specify
70the size of ring buffer and has to be a power of 2 value.
71
72There are a bunch of similarities between perf buffer
73(``BPF_MAP_TYPE_PERF_EVENT_ARRAY``) and new BPF ring buffer semantics:
74
75- variable-length records;
76- if there is no more space left in ring buffer, reservation fails, no
77  blocking;
78- memory-mappable data area for user-space applications for ease of
79  consumption and high performance;
80- epoll notifications for new incoming data;
81- but still the ability to do busy polling for new data to achieve the
82  lowest latency, if necessary.
83
84BPF ringbuf provides two sets of APIs to BPF programs:
85
86- ``bpf_ringbuf_output()`` allows to *copy* data from one place to a ring
87  buffer, similarly to ``bpf_perf_event_output()``;
88- ``bpf_ringbuf_reserve()``/``bpf_ringbuf_commit()``/``bpf_ringbuf_discard()``
89  APIs split the whole process into two steps. First, a fixed amount of space
90  is reserved. If successful, a pointer to a data inside ring buffer data
91  area is returned, which BPF programs can use similarly to a data inside
92  array/hash maps. Once ready, this piece of memory is either committed or
93  discarded. Discard is similar to commit, but makes consumer ignore the
94  record.
95
96``bpf_ringbuf_output()`` has disadvantage of incurring extra memory copy,
97because record has to be prepared in some other place first. But it allows to
98submit records of the length that's not known to verifier beforehand. It also
99closely matches ``bpf_perf_event_output()``, so will simplify migration
100significantly.
101
102``bpf_ringbuf_reserve()`` avoids the extra copy of memory by providing a memory
103pointer directly to ring buffer memory. In a lot of cases records are larger
104than BPF stack space allows, so many programs have use extra per-CPU array as
105a temporary heap for preparing sample. bpf_ringbuf_reserve() avoid this needs
106completely. But in exchange, it only allows a known constant size of memory to
107be reserved, such that verifier can verify that BPF program can't access memory
108outside its reserved record space. bpf_ringbuf_output(), while slightly slower
109due to extra memory copy, covers some use cases that are not suitable for
110``bpf_ringbuf_reserve()``.
111
112The difference between commit and discard is very small. Discard just marks
113a record as discarded, and such records are supposed to be ignored by consumer
114code. Discard is useful for some advanced use-cases, such as ensuring
115all-or-nothing multi-record submission, or emulating temporary
116``malloc()``/``free()`` within single BPF program invocation.
117
118Each reserved record is tracked by verifier through existing
119reference-tracking logic, similar to socket ref-tracking. It is thus
120impossible to reserve a record, but forget to submit (or discard) it.
121
122``bpf_ringbuf_query()`` helper allows to query various properties of ring
123buffer.  Currently 4 are supported:
124
125- ``BPF_RB_AVAIL_DATA`` returns amount of unconsumed data in ring buffer;
126- ``BPF_RB_RING_SIZE`` returns the size of ring buffer;
127- ``BPF_RB_CONS_POS``/``BPF_RB_PROD_POS`` returns current logical possition
128  of consumer/producer, respectively.
129
130Returned values are momentarily snapshots of ring buffer state and could be
131off by the time helper returns, so this should be used only for
132debugging/reporting reasons or for implementing various heuristics, that take
133into account highly-changeable nature of some of those characteristics.
134
135One such heuristic might involve more fine-grained control over poll/epoll
136notifications about new data availability in ring buffer. Together with
137``BPF_RB_NO_WAKEUP``/``BPF_RB_FORCE_WAKEUP`` flags for output/commit/discard
138helpers, it allows BPF program a high degree of control and, e.g., more
139efficient batched notifications. Default self-balancing strategy, though,
140should be adequate for most applications and will work reliable and efficiently
141already.
142
143Design and Implementation
144-------------------------
145
146This reserve/commit schema allows a natural way for multiple producers, either
147on different CPUs or even on the same CPU/in the same BPF program, to reserve
148independent records and work with them without blocking other producers. This
149means that if BPF program was interruped by another BPF program sharing the
150same ring buffer, they will both get a record reserved (provided there is
151enough space left) and can work with it and submit it independently. This
152applies to NMI context as well, except that due to using a spinlock during
153reservation, in NMI context, ``bpf_ringbuf_reserve()`` might fail to get
154a lock, in which case reservation will fail even if ring buffer is not full.
155
156The ring buffer itself internally is implemented as a power-of-2 sized
157circular buffer, with two logical and ever-increasing counters (which might
158wrap around on 32-bit architectures, that's not a problem):
159
160- consumer counter shows up to which logical position consumer consumed the
161  data;
162- producer counter denotes amount of data reserved by all producers.
163
164Each time a record is reserved, producer that "owns" the record will
165successfully advance producer counter. At that point, data is still not yet
166ready to be consumed, though. Each record has 8 byte header, which contains the
167length of reserved record, as well as two extra bits: busy bit to denote that
168record is still being worked on, and discard bit, which might be set at commit
169time if record is discarded. In the latter case, consumer is supposed to skip
170the record and move on to the next one. Record header also encodes record's
171relative offset from the beginning of ring buffer data area (in pages). This
172allows ``bpf_ringbuf_commit()``/``bpf_ringbuf_discard()`` to accept only the
173pointer to the record itself, without requiring also the pointer to ring buffer
174itself. Ring buffer memory location will be restored from record metadata
175header. This significantly simplifies verifier, as well as improving API
176usability.
177
178Producer counter increments are serialized under spinlock, so there is
179a strict ordering between reservations. Commits, on the other hand, are
180completely lockless and independent. All records become available to consumer
181in the order of reservations, but only after all previous records where
182already committed. It is thus possible for slow producers to temporarily hold
183off submitted records, that were reserved later.
184
185One interesting implementation bit, that significantly simplifies (and thus
186speeds up as well) implementation of both producers and consumers is how data
187area is mapped twice contiguously back-to-back in the virtual memory. This
188allows to not take any special measures for samples that have to wrap around
189at the end of the circular buffer data area, because the next page after the
190last data page would be first data page again, and thus the sample will still
191appear completely contiguous in virtual memory. See comment and a simple ASCII
192diagram showing this visually in ``bpf_ringbuf_area_alloc()``.
193
194Another feature that distinguishes BPF ringbuf from perf ring buffer is
195a self-pacing notifications of new data being availability.
196``bpf_ringbuf_commit()`` implementation will send a notification of new record
197being available after commit only if consumer has already caught up right up to
198the record being committed. If not, consumer still has to catch up and thus
199will see new data anyways without needing an extra poll notification.
200Benchmarks (see tools/testing/selftests/bpf/benchs/bench_ringbufs.c) show that
201this allows to achieve a very high throughput without having to resort to
202tricks like "notify only every Nth sample", which are necessary with perf
203buffer. For extreme cases, when BPF program wants more manual control of
204notifications, commit/discard/output helpers accept ``BPF_RB_NO_WAKEUP`` and
205``BPF_RB_FORCE_WAKEUP`` flags, which give full control over notifications of
206data availability, but require extra caution and diligence in using this API.
207