Em Wed, 9 Sep 2020 15:53:05 +0200 Mauro Carvalho Chehab <mchehab+huawei@xxxxxxxxxx> escreveu: > 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? Btw, make htmldocs also complains with: Documentation/bpf/ringbuf.rst:197: WARNING: Unknown target name: "bench_ringbuf.c". (this one is reported by Sphinx) > > Thanks, > Mauro Thanks, Mauro
