207 lines
11 KiB
ReStructuredText
207 lines
11 KiB
ReStructuredText
===============
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BPF ring buffer
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===============
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This document describes BPF ring buffer design, API, and implementation details.
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.. contents::
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:local:
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:depth: 2
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Motivation
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----------
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There are two distinctive motivators for this work, which are not satisfied by
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existing perf buffer, which prompted creation of a new ring buffer
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implementation.
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- more efficient memory utilization by sharing ring buffer across CPUs;
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- preserving ordering of events that happen sequentially in time, even across
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multiple CPUs (e.g., fork/exec/exit events for a task).
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These two problems are independent, but perf buffer fails to satisfy both.
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Both are a result of a choice to have per-CPU perf ring buffer. Both can be
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also solved by having an MPSC implementation of ring buffer. The ordering
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problem could technically be solved for perf buffer with some in-kernel
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counting, but given the first one requires an MPSC buffer, the same solution
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would solve the second problem automatically.
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Semantics and APIs
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------------------
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Single ring buffer is presented to BPF programs as an instance of BPF map of
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type ``BPF_MAP_TYPE_RINGBUF``. Two other alternatives considered, but
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ultimately rejected.
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One way would be to, similar to ``BPF_MAP_TYPE_PERF_EVENT_ARRAY``, make
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``BPF_MAP_TYPE_RINGBUF`` could represent an array of ring buffers, but not
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enforce "same CPU only" rule. This would be more familiar interface compatible
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with existing perf buffer use in BPF, but would fail if application needed more
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advanced logic to lookup ring buffer by arbitrary key.
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``BPF_MAP_TYPE_HASH_OF_MAPS`` addresses this with current approach.
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Additionally, given the performance of BPF ringbuf, many use cases would just
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opt into a simple single ring buffer shared among all CPUs, for which current
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approach would be an overkill.
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Another approach could introduce a new concept, alongside BPF map, to represent
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generic "container" object, which doesn't necessarily have key/value interface
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with lookup/update/delete operations. This approach would add a lot of extra
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infrastructure that has to be built for observability and verifier support. It
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would also add another concept that BPF developers would have to familiarize
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themselves with, new syntax in libbpf, etc. But then would really provide no
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additional benefits over the approach of using a map. ``BPF_MAP_TYPE_RINGBUF``
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doesn't support lookup/update/delete operations, but so doesn't few other map
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types (e.g., queue and stack; array doesn't support delete, etc).
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The approach chosen has an advantage of re-using existing BPF map
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infrastructure (introspection APIs in kernel, libbpf support, etc), being
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familiar concept (no need to teach users a new type of object in BPF program),
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and utilizing existing tooling (bpftool). For common scenario of using a single
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ring buffer for all CPUs, it's as simple and straightforward, as would be with
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a dedicated "container" object. On the other hand, by being a map, it can be
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combined with ``ARRAY_OF_MAPS`` and ``HASH_OF_MAPS`` map-in-maps to implement
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a wide variety of topologies, from one ring buffer for each CPU (e.g., as
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a replacement for perf buffer use cases), to a complicated application
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hashing/sharding of ring buffers (e.g., having a small pool of ring buffers
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with hashed task's tgid being a look up key to preserve order, but reduce
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contention).
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Key and value sizes are enforced to be zero. ``max_entries`` is used to specify
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the size of ring buffer and has to be a power of 2 value.
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There are a bunch of similarities between perf buffer
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(``BPF_MAP_TYPE_PERF_EVENT_ARRAY``) and new BPF ring buffer semantics:
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- variable-length records;
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- if there is no more space left in ring buffer, reservation fails, no
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blocking;
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- memory-mappable data area for user-space applications for ease of
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consumption and high performance;
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- epoll notifications for new incoming data;
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- but still the ability to do busy polling for new data to achieve the
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lowest latency, if necessary.
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BPF ringbuf provides two sets of APIs to BPF programs:
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- ``bpf_ringbuf_output()`` allows to *copy* data from one place to a ring
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buffer, similarly to ``bpf_perf_event_output()``;
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- ``bpf_ringbuf_reserve()``/``bpf_ringbuf_commit()``/``bpf_ringbuf_discard()``
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APIs split the whole process into two steps. First, a fixed amount of space
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is reserved. If successful, a pointer to a data inside ring buffer data
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area is returned, which BPF programs can use similarly to a data inside
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array/hash maps. Once ready, this piece of memory is either committed or
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discarded. Discard is similar to commit, but makes consumer ignore the
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record.
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``bpf_ringbuf_output()`` has disadvantage of incurring extra memory copy,
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because record has to be prepared in some other place first. But it allows to
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submit records of the length that's not known to verifier beforehand. It also
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closely matches ``bpf_perf_event_output()``, so will simplify migration
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significantly.
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``bpf_ringbuf_reserve()`` avoids the extra copy of memory by providing a memory
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pointer directly to ring buffer memory. In a lot of cases records are larger
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than BPF stack space allows, so many programs have use extra per-CPU array as
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a temporary heap for preparing sample. bpf_ringbuf_reserve() avoid this needs
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completely. But in exchange, it only allows a known constant size of memory to
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be reserved, such that verifier can verify that BPF program can't access memory
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outside its reserved record space. bpf_ringbuf_output(), while slightly slower
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due to extra memory copy, covers some use cases that are not suitable for
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``bpf_ringbuf_reserve()``.
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The difference between commit and discard is very small. Discard just marks
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a record as discarded, and such records are supposed to be ignored by consumer
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code. Discard is useful for some advanced use-cases, such as ensuring
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all-or-nothing multi-record submission, or emulating temporary
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``malloc()``/``free()`` within single BPF program invocation.
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Each reserved record is tracked by verifier through existing
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reference-tracking logic, similar to socket ref-tracking. It is thus
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impossible to reserve a record, but forget to submit (or discard) it.
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``bpf_ringbuf_query()`` helper allows to query various properties of ring
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buffer. Currently 4 are supported:
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- ``BPF_RB_AVAIL_DATA`` returns amount of unconsumed data in ring buffer;
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- ``BPF_RB_RING_SIZE`` returns the size of ring buffer;
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- ``BPF_RB_CONS_POS``/``BPF_RB_PROD_POS`` returns current logical possition
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of consumer/producer, respectively.
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Returned values are momentarily snapshots of ring buffer state and could be
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off by the time helper returns, so this should be used only for
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debugging/reporting reasons or for implementing various heuristics, that take
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into account highly-changeable nature of some of those characteristics.
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One such heuristic might involve more fine-grained control over poll/epoll
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notifications about new data availability in ring buffer. Together with
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``BPF_RB_NO_WAKEUP``/``BPF_RB_FORCE_WAKEUP`` flags for output/commit/discard
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helpers, it allows BPF program a high degree of control and, e.g., more
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efficient batched notifications. Default self-balancing strategy, though,
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should be adequate for most applications and will work reliable and efficiently
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already.
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Design and Implementation
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-------------------------
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This reserve/commit schema allows a natural way for multiple producers, either
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on different CPUs or even on the same CPU/in the same BPF program, to reserve
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independent records and work with them without blocking other producers. This
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means that if BPF program was interruped by another BPF program sharing the
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same ring buffer, they will both get a record reserved (provided there is
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enough space left) and can work with it and submit it independently. This
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applies to NMI context as well, except that due to using a spinlock during
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reservation, in NMI context, ``bpf_ringbuf_reserve()`` might fail to get
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a lock, in which case reservation will fail even if ring buffer is not full.
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The ring buffer itself internally is implemented as a power-of-2 sized
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circular buffer, with two logical and ever-increasing counters (which might
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wrap around on 32-bit architectures, that's not a problem):
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- consumer counter shows up to which logical position consumer consumed the
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data;
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- producer counter denotes amount of data reserved by all producers.
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Each time a record is reserved, producer that "owns" the record will
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successfully advance producer counter. At that point, data is still not yet
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ready to be consumed, though. Each record has 8 byte header, which contains the
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length of reserved record, as well as two extra bits: busy bit to denote that
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record is still being worked on, and discard bit, which might be set at commit
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time if record is discarded. In the latter case, consumer is supposed to skip
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the record and move on to the next one. Record header also encodes record's
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relative offset from the beginning of ring buffer data area (in pages). This
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allows ``bpf_ringbuf_commit()``/``bpf_ringbuf_discard()`` to accept only the
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pointer to the record itself, without requiring also the pointer to ring buffer
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itself. Ring buffer memory location will be restored from record metadata
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header. This significantly simplifies verifier, as well as improving API
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usability.
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Producer counter increments are serialized under spinlock, so there is
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a strict ordering between reservations. Commits, on the other hand, are
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completely lockless and independent. All records become available to consumer
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in the order of reservations, but only after all previous records where
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already committed. It is thus possible for slow producers to temporarily hold
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off submitted records, that were reserved later.
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One interesting implementation bit, that significantly simplifies (and thus
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speeds up as well) implementation of both producers and consumers is how data
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area is mapped twice contiguously back-to-back in the virtual memory. This
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allows to not take any special measures for samples that have to wrap around
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at the end of the circular buffer data area, because the next page after the
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last data page would be first data page again, and thus the sample will still
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appear completely contiguous in virtual memory. See comment and a simple ASCII
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diagram showing this visually in ``bpf_ringbuf_area_alloc()``.
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Another feature that distinguishes BPF ringbuf from perf ring buffer is
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a self-pacing notifications of new data being availability.
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``bpf_ringbuf_commit()`` implementation will send a notification of new record
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being available after commit only if consumer has already caught up right up to
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the record being committed. If not, consumer still has to catch up and thus
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will see new data anyways without needing an extra poll notification.
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Benchmarks (see tools/testing/selftests/bpf/benchs/bench_ringbufs.c) show that
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this allows to achieve a very high throughput without having to resort to
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tricks like "notify only every Nth sample", which are necessary with perf
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buffer. For extreme cases, when BPF program wants more manual control of
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notifications, commit/discard/output helpers accept ``BPF_RB_NO_WAKEUP`` and
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``BPF_RB_FORCE_WAKEUP`` flags, which give full control over notifications of
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data availability, but require extra caution and diligence in using this API.
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