Add commit description from patch #1 as a stand-alone documentation under Documentation/bpf, as it might be more convenient format, in long term perspective. Suggested-by: Stanislav Fomichev <sdf@xxxxxxxxxx> Signed-off-by: Andrii Nakryiko <andriin@xxxxxx> --- Documentation/bpf/ringbuf.txt | 191 ++++++++++++++++++++++++++++++++++ 1 file changed, 191 insertions(+) create mode 100644 Documentation/bpf/ringbuf.txt diff --git a/Documentation/bpf/ringbuf.txt b/Documentation/bpf/ringbuf.txt new file mode 100644 index 000000000000..72f0a2480db3 --- /dev/null +++ b/Documentation/bpf/ringbuf.txt @@ -0,0 +1,191 @@ +BPF ring buffer +============== + +Motivation +---------- +There are two distinctive motivators for this work, which are not satisfied by +existing perf buffer, which prompted creation of a new ring buffer +implementation. + - more efficient memory utilization by sharing ring buffer across CPUs; + - preserving ordering of events that happen sequentially in time, even + across multiple CPUs (e.g., fork/exec/exit events for a task). + +These two problems are independent, but perf buffer fails to satisfy both. +Both are a result of a choice to have per-CPU perf ring buffer. Both can be +also solved by having an MPSC implementation of ring buffer. The ordering +problem could technically be solved for perf buffer with some in-kernel +counting, but given the first one requires an MPSC buffer, the same solution +would solve the second problem automatically. + +Semantics and APIs +------------------ +Single ring buffer is presented to BPF programs as an instance of BPF map of +type BPF_MAP_TYPE_RINGBUF. Two other alternatives considered, but ultimately +rejected. + +One way would be to, similar to BPF_MAP_TYPE_PERF_EVENT_ARRAY, make +BPF_MAP_TYPE_RINGBUF could represent an array of ring buffers, but not enforce +"same CPU only" rule. This would be more familiar interface compatible with +existing perf buffer use in BPF, but would fail if application needed more +advanced logic to lookup ring buffer by arbitrary key. HASH_OF_MAPS addresses +this with current approach. Additionally, given the performance of BPF +ringbuf, many use cases would just opt into a simple single ring buffer shared +among all CPUs, for which current approach would be an overkill. + +Another approach could introduce a new concept, alongside BPF map, to +represent generic "container" object, which doesn't necessarily have key/value +interface with lookup/update/delete operations. This approach would add a lot +of extra infrastructure that has to be built for observability and verifier +support. It would also add another concept that BPF developers would have to +familiarize themselves with, new syntax in libbpf, etc. But then would really +provide no additional benefits over the approach of using a map. +BPF_MAP_TYPE_RINGBUF doesn't support lookup/update/delete operations, but so +doesn't few other map types (e.g., queue and stack; array doesn't support +delete, etc). + +The approach chosen has an advantage of re-using existing BPF map +infrastructure (introspection APIs in kernel, libbpf support, etc), being +familiar concept (no need to teach users a new type of object in BPF program), +and utilizing existing tooling (bpftool). For common scenario of using +a single ring buffer for all CPUs, it's as simple and straightforward, as +would be with a dedicated "container" object. On the other hand, by being +a map, it can be combined with ARRAY_OF_MAPS and HASH_OF_MAPS map-in-maps to +implement a wide variety of topologies, from one ring buffer for each CPU +(e.g., as a replacement for perf buffer use cases), to a complicated +application hashing/sharding of ring buffers (e.g., having a small pool of +ring buffers with hashed task's tgid being a look up key to preserve order, +but reduce contention). + +Key and value sizes are enforced to be zero. max_entries is used to specify +the size of ring buffer and has to be a power of 2 value. + +There are a bunch of similarities between perf buffer +(BPF_MAP_TYPE_PERF_EVENT_ARRAY) and new BPF ring buffer semantics: + - variable-length records; + - if there is no more space left in ring buffer, reservation fails, no + blocking; + - memory-mappable data area for user-space applications for ease of + consumption and high performance; + - epoll notifications for new incoming data; + - but still the ability to do busy polling for new data to achieve the + lowest latency, if necessary. + +BPF ringbuf provides two sets of APIs to BPF programs: + - bpf_ringbuf_output() allows to *copy* data from one place to a ring + buffer, similarly to bpf_perf_event_output(); + - bpf_ringbuf_reserve()/bpf_ringbuf_commit()/bpf_ringbuf_discard() APIs + split the whole process into two steps. First, a fixed amount of space is + reserved. If successful, a pointer to a data inside ring buffer data area + is returned, which BPF programs can use similarly to a data inside + array/hash maps. Once ready, this piece of memory is either committed or + discarded. Discard is similar to commit, but makes consumer ignore the + record. + +bpf_ringbuf_output() has disadvantage of incurring extra memory copy, because +record has to be prepared in some other place first. But it allows to submit +records of the length that's not known to verifier beforehand. It also closely +matches bpf_perf_event_output(), so will simplify migration significantly. + +bpf_ringbuf_reserve() avoids the extra copy of memory by providing a memory +pointer directly to ring buffer memory. In a lot of cases records are larger +than BPF stack space allows, so many programs have use extra per-CPU array as +a temporary heap for preparing sample. bpf_ringbuf_reserve() avoid this needs +completely. But in exchange, it only allows a known constant size of memory to +be reserved, such that verifier can verify that BPF program can't access +memory outside its reserved record space. bpf_ringbuf_output(), while slightly +slower due to extra memory copy, covers some use cases that are not suitable +for bpf_ringbuf_reserve(). + +The difference between commit and discard is very small. Discard just marks +a record as discarded, and such records are supposed to be ignored by consumer +code. Discard is useful for some advanced use-cases, such as ensuring +all-or-nothing multi-record submission, or emulating temporary malloc()/free() +within single BPF program invocation. + +Each reserved record is tracked by verifier through existing +reference-tracking logic, similar to socket ref-tracking. It is thus +impossible to reserve a record, but forget to submit (or discard) it. + +bpf_ringbuf_query() helper allows to query various properties of ring 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 of + consumer/producer, respectively. +Returned values are momentarily snapshots of ring buffer state and could be +off by the time helper returns, so this should be used only for +debugging/reporting reasons or for implementing various heuristics, that take +into account highly-changeable nature of some of those characteristics. + +One such heuristic might involve more fine-grained control over poll/epoll +notifications about new data availability in ring buffer. Together with +BPF_RB_NO_WAKEUP/BPF_RB_FORCE_WAKEUP flags for output/commit/discard helpers, +it allows BPF program a high degree of control and, e.g., more efficient +batched notifications. Default self-balancing strategy, though, should be +adequate for most applications and will work reliable and efficiently already. + +Design and implementation +------------------------- +This reserve/commit schema allows a natural way for multiple producers, either +on different CPUs or even on the same CPU/in the same BPF program, to reserve +independent records and work with them without blocking other producers. This +means that if BPF program was interruped by another BPF program sharing the +same ring buffer, they will both get a record reserved (provided there is +enough space left) and can work with it and submit it independently. This +applies to NMI context as well, except that due to using a spinlock during +reservation, in NMI context, bpf_ringbuf_reserve() might fail to get a lock, +in which case reservation will fail even if ring buffer is not full. + +The ring buffer itself internally is implemented as a power-of-2 sized +circular buffer, with two logical and ever-increasing counters (which might +wrap around on 32-bit architectures, that's not a problem): + - consumer counter shows up to which logical position consumer consumed the + data; + - producer counter denotes amount of data reserved by all producers. + +Each time a record is reserved, producer that "owns" the record will +successfully advance producer counter. At that point, data is still not yet +ready to be consumed, though. Each record has 8 byte header, which contains +the length of reserved record, as well as two extra bits: busy bit to denote +that record is still being worked on, and discard bit, which might be set at +commit time if record is discarded. In the latter case, consumer is supposed +to skip the record and move on to the next one. Record header also encodes +record's relative offset from the beginning of ring buffer data area (in +pages). This allows bpf_ringbuf_commit()/bpf_ringbuf_discard() to accept only +the pointer to the record itself, without requiring also the pointer to ring +buffer itself. Ring buffer memory location will be restored from record +metadata header. This significantly simplifies verifier, as well as improving +API usability. + +Producer counter increments are serialized under spinlock, so there is +a strict ordering between reservations. Commits, on the other hand, are +completely lockless and independent. All records become available to consumer +in the order of reservations, but only after all previous records where +already committed. It is thus possible for slow producers to temporarily hold +off submitted records, that were reserved later. + +Reservation/commit/consumer protocol is verified by litmus tests in +tools/memory-model/litmus-tests, see mpsc-rb*.litmus files. + +One interesting implementation bit, that significantly simplifies (and thus +speeds up as well) implementation of both producers and consumers is how data +area is mapped twice contiguously back-to-back in the virtual memory. This +allows to not take any special measures for samples that have to wrap around +at the end of the circular buffer data area, because the next page after the +last data page would be first data page again, and thus the sample will still +appear completely contiguous in virtual memory. See comment and a simple ASCII +diagram showing this visually in bpf_ringbuf_area_alloc(). + +Another feature that distinguishes BPF ringbuf from perf ring buffer is +a self-pacing notifications of new data being availability. +bpf_ringbuf_commit() implementation will send a notification of new record +being available after commit only if consumer has already caught up right up +to the record being committed. If not, consumer still has to catch up and thus +will see new data anyways without needing an extra poll notification. +Benchmarks (see tools/testing/selftests/bpf/benchs/bench_ringbuf.c) show that +this allows to achieve a very high throughput without having to resort to +tricks like "notify only every Nth sample", which are necessary with perf +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. -- 2.24.1
