* [PATCH v2] bpf, docs: document BPF_MAP_TYPE_HASH and variants
@ 2022-07-15 13:08 Donald Hunter
2022-07-15 16:37 ` Stanislav Fomichev
2022-07-15 17:37 ` Yonghong Song
0 siblings, 2 replies; 4+ messages in thread
From: Donald Hunter @ 2022-07-15 13:08 UTC (permalink / raw)
To: bpf, linux-doc; +Cc: Jonathan Corbet, sdf, Donald Hunter
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@gmail.com>
---
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.
+
+``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.
+
+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
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH v2] bpf, docs: document BPF_MAP_TYPE_HASH and variants
2022-07-15 13:08 [PATCH v2] bpf, docs: document BPF_MAP_TYPE_HASH and variants Donald Hunter
@ 2022-07-15 16:37 ` Stanislav Fomichev
2022-07-15 16:56 ` Stanislav Fomichev
2022-07-15 17:37 ` Yonghong Song
1 sibling, 1 reply; 4+ messages in thread
From: Stanislav Fomichev @ 2022-07-15 16:37 UTC (permalink / raw)
To: Donald Hunter; +Cc: bpf, linux-doc, Jonathan Corbet
On Fri, Jul 15, 2022 at 6:08 AM Donald Hunter <donald.hunter@gmail.com> 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@gmail.com>
> ---
> 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?
> +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
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH v2] bpf, docs: document BPF_MAP_TYPE_HASH and variants
2022-07-15 16:37 ` Stanislav Fomichev
@ 2022-07-15 16:56 ` Stanislav Fomichev
0 siblings, 0 replies; 4+ messages in thread
From: Stanislav Fomichev @ 2022-07-15 16:56 UTC (permalink / raw)
To: Donald Hunter; +Cc: bpf, linux-doc, Jonathan Corbet
On Fri, Jul 15, 2022 at 9:37 AM Stanislav Fomichev <sdf@google.com> wrote:
>
> On Fri, Jul 15, 2022 at 6:08 AM Donald Hunter <donald.hunter@gmail.com> 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@gmail.com>
> > ---
> > 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
> >
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH v2] bpf, docs: document BPF_MAP_TYPE_HASH and variants
2022-07-15 13:08 [PATCH v2] bpf, docs: document BPF_MAP_TYPE_HASH and variants Donald Hunter
2022-07-15 16:37 ` Stanislav Fomichev
@ 2022-07-15 17:37 ` Yonghong Song
1 sibling, 0 replies; 4+ messages in thread
From: Yonghong Song @ 2022-07-15 17:37 UTC (permalink / raw)
To: Donald Hunter, bpf, linux-doc; +Cc: Jonathan Corbet, sdf
On 7/15/22 6:08 AM, Donald Hunter 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@gmail.com>
> ---
> 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.
> +
> +``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
'is possible' -> 'it is possible'
> +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.
There are some potential issues related to bpf_map_get_next_key() where
if it happened the *cur_key* in bpf_map_get_next_key() is deleted, the
returned next_key will be the *first* key in the hash table. This is
an undesired behavior. So we should mention this and recommend users
to use batch-based lookup if their setup involves key deletion
intermixed with bpf_map_get_next_key(). The details can be found here:
https://lore.kernel.org/bpf/20200115184308.162644-6-brianvv@google.com
> +
> +Examples
> +========
> +
> +Please see the ``tools/testing/selftests/bpf`` directory for functional
> +examples. The sample code below demonstrates API usage.
> +
[...]
^ permalink raw reply [flat|nested] 4+ messages in thread
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2022-07-15 13:08 [PATCH v2] bpf, docs: document BPF_MAP_TYPE_HASH and variants Donald Hunter
2022-07-15 16:37 ` Stanislav Fomichev
2022-07-15 16:56 ` Stanislav Fomichev
2022-07-15 17:37 ` Yonghong Song
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