Em Fri, 29 May 2020 00:54:24 -0700 Andrii Nakryiko <andriin@xxxxxx> escreveu: > 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.rst | 209 ++++++++++++++++++++++++++++++++++ > 1 file changed, 209 insertions(+) > create mode 100644 Documentation/bpf/ringbuf.rst > > diff --git a/Documentation/bpf/ringbuf.rst b/Documentation/bpf/ringbuf.rst > new file mode 100644 > index 000000000000..75f943f0009d > --- /dev/null > +++ b/Documentation/bpf/ringbuf.rst > @@ -0,0 +1,209 @@ > +=============== > +BPF ring buffer > +=============== > + > +This document describes BPF ring buffer design, API, and implementation details. > + > +.. contents:: > + :local: > + :depth: 2 > + > +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. > +``BPF_MAP_TYPE_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 > +Documentation/litmus_tests/bpf-rb/_. Are there any missing patch that were supposed to be merged before this one: There's no Documentation/litmus_tests/bpf-rb/_. This currently causes a warning at the Kernel's building system: $ ./scripts/documentation-file-ref-check Documentation/bpf/ringbuf.rst: Documentation/litmus_tests/bpf-rb/_ (This is reported when someone calls "make htmldocs") Could you please fix this? Thanks, Mauro
