• Andrii Nakryiko's avatar
    bpf: Implement BPF ring buffer and verifier support for it · 457f4436
    Andrii Nakryiko authored
    This commit adds a new MPSC ring buffer implementation into BPF ecosystem,
    which allows multiple CPUs to submit data to a single shared ring buffer. On
    the consumption side, only single consumer is assumed.
    
    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
    Documentation/litmus-test/bpf-rb.
    
    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.
    
    Comparison to alternatives
    --------------------------
    Before considering implementing BPF ring buffer from scratch existing
    alternatives in kernel were evaluated, but didn't seem to meet the needs. They
    largely fell into few categores:
      - per-CPU buffers (perf, ftrace, etc), which don't satisfy two motivations
        outlined above (ordering and memory consumption);
      - linked list-based implementations; while some were multi-producer designs,
        consuming these from user-space would be very complicated and most
        probably not performant; memory-mapping contiguous piece of memory is
        simpler and more performant for user-space consumers;
      - io_uring is SPSC, but also requires fixed-sized elements. Naively turning
        SPSC queue into MPSC w/ lock would have subpar performance compared to
        locked reserve + lockless commit, as with BPF ring buffer. Fixed sized
        elements would be too limiting for BPF programs, given existing BPF
        programs heavily rely on variable-sized perf buffer already;
      - specialized implementations (like a new printk ring buffer, [0]) with lots
        of printk-specific limitations and implications, that didn't seem to fit
        well for intended use with BPF programs.
    
      [0] https://lwn.net/Articles/779550/Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
    Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
    Link: https://lore.kernel.org/bpf/20200529075424.3139988-2-andriin@fb.comSigned-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
    457f4436
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