Re: [PATCH bpf-next v4 2/2] docs/bpf: Add LRU internals description and graph

Bagas Sanjaya
2023-04-03T03:50:41Z

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On Sat, Apr 01, 2023 at 01:06:51PM -0700, Joe Stringer wrote:
> diff --git a/Documentation/bpf/map_hash.rst b/Documentation/bpf/map_hash.rst
> index 45d923cd16c4..ddc961f98b27 100644
> --- a/Documentation/bpf/map_hash.rst
> +++ b/Documentation/bpf/map_hash.rst
> @@ -1,5 +1,6 @@
> .. SPDX-License-Identifier: GPL-2.0-only
> .. Copyright (C) 2022 Red Hat, Inc.
> +.. Copyright (C) 2022-2023 Isovalent, Inc.
>
> ===============================================
> BPF_MAP_TYPE_HASH, with PERCPU and LRU Variants
> @@ -215,3 +216,44 @@ Userspace walking the map elements from the map declared above:
> cur_key = &next_key;
> }
> }
> +
> +Internals
> +=========
> +
> +This section of the document is targeted at Linux developers and describes
> +aspects of the map implementations that are not considered stable ABI. The
> +following details are subject to change in future versions of the kernel.
> +
> +``BPF_MAP_TYPE_LRU_HASH`` and variants
> +--------------------------------------
> +
> +Updating elements in LRU maps may trigger eviction behaviour when the capacity
> +of the map is reached. There are various steps that the update algorithm
> +attempts in order to enforce the LRU property which have increasing impacts on
> +other CPUs involved in the following operation attempts:
> +
> +- Attempt to use CPU-local state to batch operations
> +- Attempt to fetch free nodes from global lists
> +- Attempt to pull any node from a global list and remove it from the hashmap
> +- Attempt to pull any node from any CPU's list and remove it from the hashmap
> +
> +This algorithm is described visually in the following diagram. See the
> +description in commit 3a08c2fd7634 ("bpf: LRU List") for a full explanation of
> +the corresponding operations:
> +
> +.. kernel-figure:: map_lru_hash_update.dot
> + :alt: Diagram outlining the LRU eviction steps taken during map update.
> +
> + LRU hash eviction during map update for ``BPF_MAP_TYPE_LRU_HASH`` and
> + variants. See the dot file source for kernel function name code references.
> +
> +Map updates start from the oval in the top right "begin ``bpf_map_update()``"
> +and progress through the graph towards the bottom where the result may be
> +either a successful update or a failure with various error codes. The key in
> +the top right provides indicators for which locks may be involved in specific
> +operations. This is intended as a visual hint for reasoning about how map
> +contention may impact update operations, though the map type and flags may
> +impact the actual contention on those locks, based on the logic described in
> +the table above. For instance, if the map is created with type
> +``BPF_MAP_TYPE_LRU_PERCPU_HASH`` and flags ``BPF_F_NO_COMMON_LRU`` then all map
> +properties would be per-cpu.

The doc LGTM, thanks!

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>

--
An old man doll... just what I always wanted! - Clara

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