Merge branch 'bpf-ring-buffer'

Andrii Nakryiko says:

====================
Implement a new BPF ring buffer, as presented at BPF virtual conference ([0]).
It presents an alternative to perf buffer, following its semantics closely,
but allowing sharing same instance of ring buffer across multiple CPUs
efficiently.

Most patches have extensive commentary explaining various aspects, so I'll
keep cover letter short. Overall structure of the patch set:
- patch #1 adds BPF ring buffer implementation to kernel and necessary
  verifier support;
- patch #2 adds libbpf consumer implementation for BPF ringbuf;
- patch #3 adds selftest, both for single BPF ring buf use case, as well as
  using it with array/hash of maps;
- patch #4 adds extensive benchmarks and provide some analysis in commit
  message, it builds upon selftests/bpf's bench runner.
- patch #5 adds most of patch #1 commit message as a doc under
  Documentation/bpf/ringbuf.rst.

Litmus tests, validating consumer/producer protocols and memory orderings,
were moved out as discussed in [1] and are going to be posted against -rcu
tree and put under Documentation/litmus-tests/bpf-rb.

  [0] https://docs.google.com/presentation/d/18ITdg77Bj6YDOH2LghxrnFxiPWe0fAqcmJY95t_qr0w
  [1] https://lkml.org/lkml/2020/5/22/1011

v3->v4:
- fix ringbuf freeing (vunmap, __free_page); verified with a trivial loop
  creating and closing ringbuf map endlessly (Daniel);

v2->v3:
- dropped unnecessary smp_wmb() (Paul);
- verifier reference type enhancement patch was dropped (Alexei);
- better verifier message for various memory access checks (Alexei);
- clarified a bit roundup_len() bit shifting (Alexei);
- converted doc to .rst (Alexei);
- fixed warning on 32-bit arches regarding tautological ring area size check.

v1->v2:
- commit()/discard()/output() accept flags (NO_WAKEUP/FORCE_WAKEUP) (Stanislav);
- bpf_ringbuf_query() added, returning available data size, ringbuf size,
  consumer/producer positions, needed to implement smarter notification policy
  (Stanislav);
- added ringbuf UAPI constants to include/uapi/linux/bpf.h (Jonathan);
- fixed sample size check, added proper ringbuf size check (Jonathan, Alexei);
- wake_up_all() is done through irq_work (Alexei);
- consistent use of smp_load_acquire/smp_store_release, no
  READ_ONCE/WRITE_ONCE (Alexei);
- added Documentation/bpf/ringbuf.txt (Stanislav);
- updated litmus test with smp_load_acquire/smp_store_release changes;
- added ring_buffer__consume() API to libbpf for busy-polling;
- ring_buffer__poll() on success returns number of records consumed;
- fixed EPOLL notifications, don't assume available data, done similarly to
  perfbuf's implementation;
- both ringbuf and perfbuf now have --rb-sampled mode, instead of
  pb-raw/pb-custom mode, updated benchmark results;
- extended ringbuf selftests to validate epoll logic/manual notification
  logic, as well as bpf_ringbuf_query().
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

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/_.
+
+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.

include/linux/bpf.h

@@ -90,6 +90,8 @@ struct bpf_map_ops {
 	int (*map_direct_value_meta)(const struct bpf_map *map,
 				     u64 imm, u32 *off);
 	int (*map_mmap)(struct bpf_map *map, struct vm_area_struct *vma);
+	__poll_t (*map_poll)(struct bpf_map *map, struct file *filp,
+			     struct poll_table_struct *pts);
 };
 
 struct bpf_map_memory {
@@ -244,6 +246,9 @@ enum bpf_arg_type {
 	ARG_PTR_TO_LONG,	/* pointer to long */
 	ARG_PTR_TO_SOCKET,	/* pointer to bpf_sock (fullsock) */
 	ARG_PTR_TO_BTF_ID,	/* pointer to in-kernel struct */
+	ARG_PTR_TO_ALLOC_MEM,	/* pointer to dynamically allocated memory */
+	ARG_PTR_TO_ALLOC_MEM_OR_NULL,	/* pointer to dynamically allocated memory or NULL */
+	ARG_CONST_ALLOC_SIZE_OR_ZERO,	/* number of allocated bytes requested */
 };
 
 /* type of values returned from helper functions */
@@ -255,6 +260,7 @@ enum bpf_return_type {
 	RET_PTR_TO_SOCKET_OR_NULL,	/* returns a pointer to a socket or NULL */
 	RET_PTR_TO_TCP_SOCK_OR_NULL,	/* returns a pointer to a tcp_sock or NULL */
 	RET_PTR_TO_SOCK_COMMON_OR_NULL,	/* returns a pointer to a sock_common or NULL */
+	RET_PTR_TO_ALLOC_MEM_OR_NULL,	/* returns a pointer to dynamically allocated memory or NULL */
 };
 
 /* eBPF function prototype used by verifier to allow BPF_CALLs from eBPF programs
@@ -322,6 +328,8 @@ enum bpf_reg_type {
 	PTR_TO_XDP_SOCK,	 /* reg points to struct xdp_sock */
 	PTR_TO_BTF_ID,		 /* reg points to kernel struct */
 	PTR_TO_BTF_ID_OR_NULL,	 /* reg points to kernel struct or NULL */
+	PTR_TO_MEM,		 /* reg points to valid memory region */
+	PTR_TO_MEM_OR_NULL,	 /* reg points to valid memory region or NULL */
 };
 
 /* The information passed from prog-specific *_is_valid_access
@@ -1611,6 +1619,11 @@ extern const struct bpf_func_proto bpf_tcp_sock_proto;
 extern const struct bpf_func_proto bpf_jiffies64_proto;
 extern const struct bpf_func_proto bpf_get_ns_current_pid_tgid_proto;
 extern const struct bpf_func_proto bpf_event_output_data_proto;
+extern const struct bpf_func_proto bpf_ringbuf_output_proto;
+extern const struct bpf_func_proto bpf_ringbuf_reserve_proto;
+extern const struct bpf_func_proto bpf_ringbuf_submit_proto;
+extern const struct bpf_func_proto bpf_ringbuf_discard_proto;
+extern const struct bpf_func_proto bpf_ringbuf_query_proto;
 
 const struct bpf_func_proto *bpf_tracing_func_proto(
 	enum bpf_func_id func_id, const struct bpf_prog *prog);

include/linux/bpf_types.h

@@ -118,6 +118,7 @@ BPF_MAP_TYPE(BPF_MAP_TYPE_STACK, stack_map_ops)
 #if defined(CONFIG_BPF_JIT)
 BPF_MAP_TYPE(BPF_MAP_TYPE_STRUCT_OPS, bpf_struct_ops_map_ops)
 #endif
+BPF_MAP_TYPE(BPF_MAP_TYPE_RINGBUF, ringbuf_map_ops)
 
 BPF_LINK_TYPE(BPF_LINK_TYPE_RAW_TRACEPOINT, raw_tracepoint)
 BPF_LINK_TYPE(BPF_LINK_TYPE_TRACING, tracing)

include/linux/bpf_verifier.h

@@ -54,6 +54,8 @@ struct bpf_reg_state {
 
 		u32 btf_id; /* for PTR_TO_BTF_ID */
 
+		u32 mem_size; /* for PTR_TO_MEM | PTR_TO_MEM_OR_NULL */
+
 		/* Max size from any of the above. */
 		unsigned long raw;
 	};
@@ -63,6 +65,8 @@ struct bpf_reg_state {
 	 * offset, so they can share range knowledge.
 	 * For PTR_TO_MAP_VALUE_OR_NULL this is used to share which map value we
 	 * came from, when one is tested for != NULL.
+	 * For PTR_TO_MEM_OR_NULL this is used to identify memory allocation
+	 * for the purpose of tracking that it's freed.
 	 * For PTR_TO_SOCKET this is used to share which pointers retain the
 	 * same reference to the socket, to determine proper reference freeing.
 	 */

include/uapi/linux/bpf.h

@@ -147,6 +147,7 @@ enum bpf_map_type {
 	BPF_MAP_TYPE_SK_STORAGE,
 	BPF_MAP_TYPE_DEVMAP_HASH,
 	BPF_MAP_TYPE_STRUCT_OPS,
+	BPF_MAP_TYPE_RINGBUF,
 };
 
 /* Note that tracing related programs such as
@@ -3157,6 +3158,59 @@ union bpf_attr {
  *		**bpf_sk_cgroup_id**\ ().
  *	Return
  *		The id is returned or 0 in case the id could not be retrieved.
+ *
+ * void *bpf_ringbuf_output(void *ringbuf, void *data, u64 size, u64 flags)
+ * 	Description
+ * 		Copy *size* bytes from *data* into a ring buffer *ringbuf*.
+ * 		If BPF_RB_NO_WAKEUP is specified in *flags*, no notification of
+ * 		new data availability is sent.
+ * 		IF BPF_RB_FORCE_WAKEUP is specified in *flags*, notification of
+ * 		new data availability is sent unconditionally.
+ * 	Return
+ * 		0, on success;
+ * 		< 0, on error.
+ *
+ * void *bpf_ringbuf_reserve(void *ringbuf, u64 size, u64 flags)
+ * 	Description
+ * 		Reserve *size* bytes of payload in a ring buffer *ringbuf*.
+ * 	Return
+ * 		Valid pointer with *size* bytes of memory available; NULL,
+ * 		otherwise.
+ *
+ * void bpf_ringbuf_submit(void *data, u64 flags)
+ * 	Description
+ * 		Submit reserved ring buffer sample, pointed to by *data*.
+ * 		If BPF_RB_NO_WAKEUP is specified in *flags*, no notification of
+ * 		new data availability is sent.
+ * 		IF BPF_RB_FORCE_WAKEUP is specified in *flags*, notification of
+ * 		new data availability is sent unconditionally.
+ * 	Return
+ * 		Nothing. Always succeeds.
+ *
+ * void bpf_ringbuf_discard(void *data, u64 flags)
+ * 	Description
+ * 		Discard reserved ring buffer sample, pointed to by *data*.
+ * 		If BPF_RB_NO_WAKEUP is specified in *flags*, no notification of
+ * 		new data availability is sent.
+ * 		IF BPF_RB_FORCE_WAKEUP is specified in *flags*, notification of
+ * 		new data availability is sent unconditionally.
+ * 	Return
+ * 		Nothing. Always succeeds.
+ *
+ * u64 bpf_ringbuf_query(void *ringbuf, u64 flags)
+ *	Description
+ *		Query various characteristics of provided ring buffer. What
+ *		exactly is queries is determined by *flags*:
+ *		  - BPF_RB_AVAIL_DATA - amount of data not yet consumed;
+ *		  - BPF_RB_RING_SIZE - the size of ring buffer;
+ *		  - BPF_RB_CONS_POS - consumer position (can wrap around);
+ *		  - BPF_RB_PROD_POS - producer(s) position (can wrap around);
+ *		Data returned is just a momentary snapshots of actual values
+ *		and could be inaccurate, so this facility should be used to
+ *		power heuristics and for reporting, not to make 100% correct
+ *		calculation.
+ *	Return
+ *		Requested value, or 0, if flags are not recognized.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3288,7 +3342,12 @@ union bpf_attr {
 	FN(seq_printf),			\
 	FN(seq_write),			\
 	FN(sk_cgroup_id),		\
-	FN(sk_ancestor_cgroup_id),
+	FN(sk_ancestor_cgroup_id),	\
+	FN(ringbuf_output),		\
+	FN(ringbuf_reserve),		\
+	FN(ringbuf_submit),		\
+	FN(ringbuf_discard),		\
+	FN(ringbuf_query),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
@@ -3398,6 +3457,29 @@ enum {
 	BPF_F_GET_BRANCH_RECORDS_SIZE	= (1ULL << 0),
 };
 
+/* BPF_FUNC_bpf_ringbuf_commit, BPF_FUNC_bpf_ringbuf_discard, and
+ * BPF_FUNC_bpf_ringbuf_output flags.
+ */
+enum {
+	BPF_RB_NO_WAKEUP		= (1ULL << 0),
+	BPF_RB_FORCE_WAKEUP		= (1ULL << 1),
+};
+
+/* BPF_FUNC_bpf_ringbuf_query flags */
+enum {
+	BPF_RB_AVAIL_DATA = 0,
+	BPF_RB_RING_SIZE = 1,
+	BPF_RB_CONS_POS = 2,
+	BPF_RB_PROD_POS = 3,
+};
+
+/* BPF ring buffer constants */
+enum {
+	BPF_RINGBUF_BUSY_BIT		= (1U << 31),
+	BPF_RINGBUF_DISCARD_BIT		= (1U << 30),
+	BPF_RINGBUF_HDR_SZ		= 8,
+};
+
 /* Mode for BPF_FUNC_skb_adjust_room helper. */
 enum bpf_adj_room_mode {
 	BPF_ADJ_ROOM_NET,