On Fri, Jul 15, 2022 at 9:37 AM Stanislav Fomichev <sdf@xxxxxxxxxx> wrote:
>
> On Fri, Jul 15, 2022 at 6:08 AM Donald Hunter <donald.hunter@xxxxxxxxx> wrote:
> >
> > Add documentation for BPF_MAP_TYPE_HASH including kernel version
> > introduced, usage and examples. Document BPF_MAP_TYPE_PERCPU_HASH,
> > BPF_MAP_TYPE_LRU_HASH and BPF_MAP_TYPE_LRU_PERCPU_HASH variations.
> >
> > Note that this file is included in the BPF documentation by the glob in
> > Documentation/bpf/maps.rst
> >
> > Signed-off-by: Donald Hunter <donald.hunter@xxxxxxxxx>
> > ---
> > Documentation/bpf/map_hash.rst | 181 +++++++++++++++++++++++++++++++++
> > 1 file changed, 181 insertions(+)
> > create mode 100644 Documentation/bpf/map_hash.rst
> >
> > diff --git a/Documentation/bpf/map_hash.rst b/Documentation/bpf/map_hash.rst
> > new file mode 100644
> > index 000000000000..d9e33152dae5
> > --- /dev/null
> > +++ b/Documentation/bpf/map_hash.rst
> > @@ -0,0 +1,181 @@
> > +.. SPDX-License-Identifier: GPL-2.0-only
> > +.. Copyright (C) 2022 Red Hat, Inc.
> > +
> > +===============================================
> > +BPF_MAP_TYPE_HASH, with PERCPU and LRU Variants
> > +===============================================
> > +
> > +.. note::
> > + - ``BPF_MAP_TYPE_HASH`` was introduced in kernel version 3.19
> > + - ``BPF_MAP_TYPE_PERCPU_HASH`` was introduced in version 4.6
> > + - Both ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
> > + were introduced in version 4.10
> > +
> > +``BPF_MAP_TYPE_HASH`` and ``BPF_MAP_TYPE_PERCPU_HASH`` provide general
> > +purpose hash map storage. Both the key and the value can be structs,
> > +allowing for composite keys and values.
> > +
> > +The kernel is responsible for allocating and freeing key/value pairs, up
> > +to the max_entries limit that you specify. Hash maps use pre-allocation
> > +of hash table elements by default. The ``BPF_F_NO_PREALLOC`` flag can be
> > +used to disable pre-allocation when it is to memory expensive.
>
> nit:
> to memory expensive -> too memory expensive?
>
> > +``BPF_MAP_TYPE_PERCPU_HASH`` provides a separate value slot per
> > +CPU. The per-cpu values are stored internally in an array.
> > +
> > +The ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
> > +variants add LRU semantics to their respective hash tables. An LRU hash
> > +will automatically evict the least recently used entries when the hash
> > +table reaches capacity. An LRU hash maintains an internal LRU list that
> > +is used to select elements for eviction. This internal LRU list is
> > +shared across CPUs but it is possible to request a per CPU LRU list with
> > +the ``BPF_F_NO_COMMON_LRU`` flag when calling ``bpf_map_create``.
> > +
> > +Usage
> > +=====
> > +
> > +.. c:function::
> > + long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
> > +
> > +Hash entries can be added or updated using the ``bpf_map_update_elem()``
> > +helper. This helper replaces existing elements atomically. The ``flags``
> > +parameter can be used to control the update behaviour:
> > +
> > +- ``BPF_ANY`` will create a new element or update an existing element
> > +- ``BPF_NOTEXIST`` will create a new element only if one did not already
> > + exist
> > +- ``BPF_EXIST`` will update an existing element
> > +
> > +``bpf_map_update_elem()`` returns 0 on success, or negative error in
> > +case of failure.
> > +
> > +.. c:function::
> > + void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
> > +
> > +Hash entries can be retrieved using the ``bpf_map_lookup_elem()``
> > +helper. This helper returns a pointer to the value associated with
> > +``key``, or ``NULL`` if no entry was found.
> > +
> > +.. c:function::
> > + long bpf_map_delete_elem(struct bpf_map *map, const void *key)
> > +
> > +Hash entries can be deleted using the ``bpf_map_delete_elem()``
> > +helper. This helper will return 0 on success, or negative error in case
> > +of failure.
> > +
> > +Per CPU Hashes
> > +--------------
> > +
> > +For ``BPF_MAP_TYPE_PERCPU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
> > +the ``bpf_map_update_elem()`` and ``bpf_map_lookup_elem()`` helpers
> > +automatically access the hash slot for the current CPU.
> > +
> > +.. c:function::
> > + void *bpf_map_lookup_percpu_elem(struct bpf_map *map, const void *key, u32 cpu)
> > +
> > +The ``bpf_map_lookup_percpu_elem()`` helper can be used to lookup the
> > +value in the hash slot for a specific CPU. Returns value associated with
> > +``key`` on ``cpu`` , or ``NULL`` if no entry was found or ``cpu`` is
> > +invalid.
> > +
> > +Concurrency
> > +-----------
> > +
> > +Values stored in ``BPF_MAP_TYPE_HASH`` can be accessed concurrently by
> > +programs running on different CPUs. Since Kernel version 5.1, the BPF
> > +infrastructure provides ``struct bpf_spin_lock`` to synchronize access.
> > +See ``tools/testing/selftests/bpf/progs/test_spin_lock.c``.
> > +
> > +Userspace
> > +---------
> > +
> > +.. c:function::
> > + int bpf_map_get_next_key (int fd, const void *cur_key, void *next_key)
> > +
> > +In userspace, is possible to iterate through the keys of a hash using
> > +the ``bpf_map_get_next_key()`` function. The first key can be fetched by
> > +calling ``bpf_map_get_next_key()`` with ``cur_key`` set to
> > +``NULL``. Subsequent calls will fetch the next key that follows the
> > +current key. ``bpf_map_get_next_key()`` returns 0 on success, -ENOENT if
> > +cur_key is the last key in the hash, or negative error in case of
> > +failure.
> > +
> > +Examples
> > +========
> > +
> > +Please see the ``tools/testing/selftests/bpf`` directory for functional
> > +examples. The sample code below demonstrates API usage.
>
> I'd still personally prefer if you link to some existing test instead
> of having a code sample here. This is the code nobody will keep
> healthy here. Why do you think it's beneficial? Why not link to some
> test case or some sample?
Ooops, sorry, I somehow missed your original reply, found it. Yeah, if
you want the examples, let's intentionally make them incomplete and
small/non-copy-pastable? Otherwise, my concern is that people will
copy the example, get some compile error and get more puzzled than
without the example.
> > +This example shows how to declare an LRU Hash with a struct key and a
> > +struct value.
> > +
> > +.. code-block:: c
> > +
> > + #include <linux/bpf.h>
> > + #include <bpf/bpf_helpers.h>
> > +
> > + struct key {
> > + __u32 srcip;
> > + };
> > +
> > + struct value {
> > + __u64 packets;
> > + __u64 bytes;
> > + };
> > +
> > + struct {
> > + __uint(type, BPF_MAP_TYPE_LRU_HASH);
> > + __uint(max_entries, 32);
> > + __type(key, struct key);
> > + __type(value, struct value);
> > + } packet_stats SEC(".maps");
> > +
> > +This example shows how to create or update hash values using atomic
> > +instructions:
> > +
> > +.. code-block:: c
> > +
> > + static inline void (__u32 srcip, int bytes)
> > + {
> > + struct key key = {
> > + .srcip = srcip
> > + };
> > + struct value *value = bpf_map_lookup_elem(&packet_stats, &key);
> > + if (value) {
> > + __sync_fetch_and_add(&value->packets, 1);
> > + __sync_fetch_and_add(&value->bytes, bytes);
> > + } else {
> > + struct value newval = { 1, bytes };
> > + bpf_map_update_elem(&packet_stats, &key, &newval, BPF_NOEXIST);
> > + }
> > + }
> > +
> > +Userspace walking the map elements from the map declared above:
> > +
> > +.. code-block:: c
> > +
> > + #include <bpf/libbpf.h>
> > + #include <bpf/bpf.h>
> > +
> > + static void walk_hash_elements(int map_fd)
> > + {
> > + struct key *cur_key = NULL;
> > + struct key next_key;
> > + int next;
> > + do {
> > + // error checking omitted
> > + next = bpf_map_get_next_key(stats_fd, cur_key, &next_key);
> > + if (next == -ENOENT)
> > + break;
> > +
> > + struct in_addr src_addr = {
> > + .s_addr = next_key.srcip
> > + };
> > + struct value value;
> > + int ret = bpf_map_lookup_elem(stats_fd, &next_key, &value);
> > +
> > + // Use key and value here
> > +
> > + cur_key = &next_key;
> > + } while (next == 0);
> > + }
> > --
> > 2.35.1
> >
