260 lines
9.0 KiB
ReStructuredText
260 lines
9.0 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0-only
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.. Copyright (C) 2022 Red Hat, Inc.
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.. Copyright (C) 2022-2023 Isovalent, Inc.
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===============================================
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BPF_MAP_TYPE_HASH, with PERCPU and LRU Variants
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===============================================
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.. note::
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- ``BPF_MAP_TYPE_HASH`` was introduced in kernel version 3.19
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- ``BPF_MAP_TYPE_PERCPU_HASH`` was introduced in version 4.6
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- Both ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
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were introduced in version 4.10
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``BPF_MAP_TYPE_HASH`` and ``BPF_MAP_TYPE_PERCPU_HASH`` provide general
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purpose hash map storage. Both the key and the value can be structs,
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allowing for composite keys and values.
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The kernel is responsible for allocating and freeing key/value pairs, up
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to the max_entries limit that you specify. Hash maps use pre-allocation
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of hash table elements by default. The ``BPF_F_NO_PREALLOC`` flag can be
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used to disable pre-allocation when it is too memory expensive.
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``BPF_MAP_TYPE_PERCPU_HASH`` provides a separate value slot per
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CPU. The per-cpu values are stored internally in an array.
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The ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
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variants add LRU semantics to their respective hash tables. An LRU hash
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will automatically evict the least recently used entries when the hash
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table reaches capacity. An LRU hash maintains an internal LRU list that
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is used to select elements for eviction. This internal LRU list is
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shared across CPUs but it is possible to request a per CPU LRU list with
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the ``BPF_F_NO_COMMON_LRU`` flag when calling ``bpf_map_create``. The
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following table outlines the properties of LRU maps depending on the a
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map type and the flags used to create the map.
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======================== ========================= ================================
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Flag ``BPF_MAP_TYPE_LRU_HASH`` ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
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======================== ========================= ================================
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**BPF_F_NO_COMMON_LRU** Per-CPU LRU, global map Per-CPU LRU, per-cpu map
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**!BPF_F_NO_COMMON_LRU** Global LRU, global map Global LRU, per-cpu map
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======================== ========================= ================================
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Usage
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=====
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Kernel BPF
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----------
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bpf_map_update_elem()
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~~~~~~~~~~~~~~~~~~~~~
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.. code-block:: c
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long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
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Hash entries can be added or updated using the ``bpf_map_update_elem()``
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helper. This helper replaces existing elements atomically. The ``flags``
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parameter can be used to control the update behaviour:
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- ``BPF_ANY`` will create a new element or update an existing element
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- ``BPF_NOEXIST`` will create a new element only if one did not already
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exist
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- ``BPF_EXIST`` will update an existing element
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``bpf_map_update_elem()`` returns 0 on success, or negative error in
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case of failure.
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bpf_map_lookup_elem()
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~~~~~~~~~~~~~~~~~~~~~
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.. code-block:: c
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void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
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Hash entries can be retrieved using the ``bpf_map_lookup_elem()``
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helper. This helper returns a pointer to the value associated with
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``key``, or ``NULL`` if no entry was found.
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bpf_map_delete_elem()
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~~~~~~~~~~~~~~~~~~~~~
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.. code-block:: c
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long bpf_map_delete_elem(struct bpf_map *map, const void *key)
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Hash entries can be deleted using the ``bpf_map_delete_elem()``
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helper. This helper will return 0 on success, or negative error in case
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of failure.
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Per CPU Hashes
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--------------
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For ``BPF_MAP_TYPE_PERCPU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
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the ``bpf_map_update_elem()`` and ``bpf_map_lookup_elem()`` helpers
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automatically access the hash slot for the current CPU.
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bpf_map_lookup_percpu_elem()
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. code-block:: c
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void *bpf_map_lookup_percpu_elem(struct bpf_map *map, const void *key, u32 cpu)
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The ``bpf_map_lookup_percpu_elem()`` helper can be used to lookup the
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value in the hash slot for a specific CPU. Returns value associated with
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``key`` on ``cpu`` , or ``NULL`` if no entry was found or ``cpu`` is
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invalid.
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Concurrency
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-----------
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Values stored in ``BPF_MAP_TYPE_HASH`` can be accessed concurrently by
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programs running on different CPUs. Since Kernel version 5.1, the BPF
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infrastructure provides ``struct bpf_spin_lock`` to synchronise access.
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See ``tools/testing/selftests/bpf/progs/test_spin_lock.c``.
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Userspace
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---------
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bpf_map_get_next_key()
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~~~~~~~~~~~~~~~~~~~~~~
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.. code-block:: c
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int bpf_map_get_next_key(int fd, const void *cur_key, void *next_key)
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In userspace, it is possible to iterate through the keys of a hash using
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libbpf's ``bpf_map_get_next_key()`` function. The first key can be fetched by
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calling ``bpf_map_get_next_key()`` with ``cur_key`` set to
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``NULL``. Subsequent calls will fetch the next key that follows the
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current key. ``bpf_map_get_next_key()`` returns 0 on success, -ENOENT if
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cur_key is the last key in the hash, or negative error in case of
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failure.
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Note that if ``cur_key`` gets deleted then ``bpf_map_get_next_key()``
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will instead return the *first* key in the hash table which is
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undesirable. It is recommended to use batched lookup if there is going
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to be key deletion intermixed with ``bpf_map_get_next_key()``.
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Examples
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========
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Please see the ``tools/testing/selftests/bpf`` directory for functional
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examples. The code snippets below demonstrates API usage.
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This example shows how to declare an LRU Hash with a struct key and a
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struct value.
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.. code-block:: c
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#include <linux/bpf.h>
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#include <bpf/bpf_helpers.h>
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struct key {
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__u32 srcip;
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};
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struct value {
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__u64 packets;
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__u64 bytes;
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};
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struct {
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__uint(type, BPF_MAP_TYPE_LRU_HASH);
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__uint(max_entries, 32);
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__type(key, struct key);
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__type(value, struct value);
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} packet_stats SEC(".maps");
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This example shows how to create or update hash values using atomic
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instructions:
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.. code-block:: c
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static void update_stats(__u32 srcip, int bytes)
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{
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struct key key = {
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.srcip = srcip,
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};
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struct value *value = bpf_map_lookup_elem(&packet_stats, &key);
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if (value) {
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__sync_fetch_and_add(&value->packets, 1);
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__sync_fetch_and_add(&value->bytes, bytes);
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} else {
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struct value newval = { 1, bytes };
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bpf_map_update_elem(&packet_stats, &key, &newval, BPF_NOEXIST);
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}
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}
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Userspace walking the map elements from the map declared above:
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.. code-block:: c
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#include <bpf/libbpf.h>
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#include <bpf/bpf.h>
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static void walk_hash_elements(int map_fd)
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{
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struct key *cur_key = NULL;
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struct key next_key;
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struct value value;
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int err;
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for (;;) {
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err = bpf_map_get_next_key(map_fd, cur_key, &next_key);
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if (err)
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break;
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bpf_map_lookup_elem(map_fd, &next_key, &value);
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// Use key and value here
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cur_key = &next_key;
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}
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}
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Internals
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=========
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This section of the document is targeted at Linux developers and describes
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aspects of the map implementations that are not considered stable ABI. The
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following details are subject to change in future versions of the kernel.
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``BPF_MAP_TYPE_LRU_HASH`` and variants
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--------------------------------------
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Updating elements in LRU maps may trigger eviction behaviour when the capacity
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of the map is reached. There are various steps that the update algorithm
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attempts in order to enforce the LRU property which have increasing impacts on
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other CPUs involved in the following operation attempts:
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- Attempt to use CPU-local state to batch operations
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- Attempt to fetch free nodes from global lists
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- Attempt to pull any node from a global list and remove it from the hashmap
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- Attempt to pull any node from any CPU's list and remove it from the hashmap
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This algorithm is described visually in the following diagram. See the
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description in commit 3a08c2fd7634 ("bpf: LRU List") for a full explanation of
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the corresponding operations:
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.. kernel-figure:: map_lru_hash_update.dot
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:alt: Diagram outlining the LRU eviction steps taken during map update.
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LRU hash eviction during map update for ``BPF_MAP_TYPE_LRU_HASH`` and
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variants. See the dot file source for kernel function name code references.
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Map updates start from the oval in the top right "begin ``bpf_map_update()``"
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and progress through the graph towards the bottom where the result may be
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either a successful update or a failure with various error codes. The key in
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the top right provides indicators for which locks may be involved in specific
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operations. This is intended as a visual hint for reasoning about how map
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contention may impact update operations, though the map type and flags may
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impact the actual contention on those locks, based on the logic described in
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the table above. For instance, if the map is created with type
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``BPF_MAP_TYPE_LRU_PERCPU_HASH`` and flags ``BPF_F_NO_COMMON_LRU`` then all map
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properties would be per-cpu.
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