docs/bpf: Add LRU internals description and graph

Extend the bpf hashmap docs to include a brief description of the
internals of the LRU map type (setting appropriate API expectations),
including the original commit message from Martin and a variant on the
graph that I had presented during my Linux Plumbers Conference 2022
talk on "Pressure feedback for LRU map types"[0].

The node names in the dot file correspond roughly to the functions
where the logic for those decisions or steps is defined, to help
curious developers to cross-reference and update this logic if the
details of the LRU implementation ever differ from this description.

[0] https://lpc.events/event/16/contributions/1368/

Signed-off-by: Joe Stringer <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Bagas Sanjaya <[e-mail]>
Acked-by: John Fastabend <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

Documentation/bpf/map_hash.rst

···
 .. 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
···
                     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.

Documentation/bpf/map_lru_hash_update.dot

···
+// SPDX-License-Identifier: GPL-2.0-only
+// Copyright (C) 2022-2023 Isovalent, Inc.
+digraph {
+  node [colorscheme=accent4,style=filled] # Apply colorscheme to all nodes
+  graph [splines=ortho, nodesep=1]
+
+  subgraph cluster_key {
+    label = "Key\n(locks held during operation)";
+    rankdir = TB;
+
+    remote_lock [shape=rectangle,fillcolor=4,label="remote CPU LRU lock"]
+    hash_lock [shape=rectangle,fillcolor=3,label="hashtab lock"]
+    lru_lock [shape=rectangle,fillcolor=2,label="LRU lock"]
+    local_lock [shape=rectangle,fillcolor=1,label="local CPU LRU lock"]
+    no_lock [shape=rectangle,label="no locks held"]
+  }
+
+  begin [shape=oval,label="begin\nbpf_map_update()"]
+
+  // Nodes below with an 'fn_' prefix are roughly labeled by the C function
+  // names that initiate the corresponding logic in kernel/bpf/bpf_lru_list.c.
+  // Number suffixes and errno suffixes handle subsections of the corresponding
+  // logic in the function as of the writing of this dot.
+
+  // cf. __local_list_pop_free() / bpf_percpu_lru_pop_free()
+  local_freelist_check [shape=diamond,fillcolor=1,
+    label="Local freelist\nnode available?"];
+  use_local_node [shape=rectangle,
+    label="Use node owned\nby this CPU"]
+
+  // cf. bpf_lru_pop_free()
+  common_lru_check [shape=diamond,
+    label="Map created with\ncommon LRU?\n(!BPF_F_NO_COMMON_LRU)"];
+
+  fn_bpf_lru_list_pop_free_to_local [shape=rectangle,fillcolor=2,
+    label="Flush local pending,
+    Rotate Global list, move
+    LOCAL_FREE_TARGET
+    from global -> local"]
+  // Also corresponds to:
+  // fn__local_list_flush()
+  // fn_bpf_lru_list_rotate()
+  fn___bpf_lru_node_move_to_free[shape=diamond,fillcolor=2,
+    label="Able to free\nLOCAL_FREE_TARGET\nnodes?"]
+
+  fn___bpf_lru_list_shrink_inactive [shape=rectangle,fillcolor=3,
+    label="Shrink inactive list
+      up to remaining
+      LOCAL_FREE_TARGET
+      (global LRU -> local)"]
+  fn___bpf_lru_list_shrink [shape=diamond,fillcolor=2,
+    label="> 0 entries in\nlocal free list?"]
+  fn___bpf_lru_list_shrink2 [shape=rectangle,fillcolor=2,
+    label="Steal one node from
+      inactive, or if empty,
+      from active global list"]
+  fn___bpf_lru_list_shrink3 [shape=rectangle,fillcolor=3,
+    label="Try to remove\nnode from hashtab"]
+
+  local_freelist_check2 [shape=diamond,label="Htab removal\nsuccessful?"]
+  common_lru_check2 [shape=diamond,
+    label="Map created with\ncommon LRU?\n(!BPF_F_NO_COMMON_LRU)"];
+
+  subgraph cluster_remote_lock {
+    label = "Iterate through CPUs\n(start from current)";
+    style = dashed;
+    rankdir=LR;
+
+    local_freelist_check5 [shape=diamond,fillcolor=4,
+      label="Steal a node from\nper-cpu freelist?"]
+    local_freelist_check6 [shape=rectangle,fillcolor=4,
+      label="Steal a node from
+        (1) Unreferenced pending, or
+        (2) Any pending node"]
+    local_freelist_check7 [shape=rectangle,fillcolor=3,
+      label="Try to remove\nnode from hashtab"]
+    fn_htab_lru_map_update_elem [shape=diamond,
+      label="Stole node\nfrom remote\nCPU?"]
+    fn_htab_lru_map_update_elem2 [shape=diamond,label="Iterated\nall CPUs?"]
+    // Also corresponds to:
+    // use_local_node()
+    // fn__local_list_pop_pending()
+  }
+
+  fn_bpf_lru_list_pop_free_to_local2 [shape=rectangle,
+    label="Use node that was\nnot recently referenced"]
+  local_freelist_check4 [shape=rectangle,
+    label="Use node that was\nactively referenced\nin global list"]
+  fn_htab_lru_map_update_elem_ENOMEM [shape=oval,label="return -ENOMEM"]
+  fn_htab_lru_map_update_elem3 [shape=rectangle,
+    label="Use node that was\nactively referenced\nin (another?) CPU's cache"]
+  fn_htab_lru_map_update_elem4 [shape=rectangle,fillcolor=3,
+    label="Update hashmap\nwith new element"]
+  fn_htab_lru_map_update_elem5 [shape=oval,label="return 0"]
+  fn_htab_lru_map_update_elem_EBUSY [shape=oval,label="return -EBUSY"]
+  fn_htab_lru_map_update_elem_EEXIST [shape=oval,label="return -EEXIST"]
+  fn_htab_lru_map_update_elem_ENOENT [shape=oval,label="return -ENOENT"]
+
+  begin -> local_freelist_check
+  local_freelist_check -> use_local_node [xlabel="Y"]
+  local_freelist_check -> common_lru_check [xlabel="N"]
+  common_lru_check -> fn_bpf_lru_list_pop_free_to_local [xlabel="Y"]
+  common_lru_check -> fn___bpf_lru_list_shrink_inactive [xlabel="N"]
+  fn_bpf_lru_list_pop_free_to_local -> fn___bpf_lru_node_move_to_free
+  fn___bpf_lru_node_move_to_free ->
+    fn_bpf_lru_list_pop_free_to_local2 [xlabel="Y"]
+  fn___bpf_lru_node_move_to_free ->
+    fn___bpf_lru_list_shrink_inactive [xlabel="N"]
+  fn___bpf_lru_list_shrink_inactive -> fn___bpf_lru_list_shrink
+  fn___bpf_lru_list_shrink -> fn_bpf_lru_list_pop_free_to_local2 [xlabel = "Y"]
+  fn___bpf_lru_list_shrink -> fn___bpf_lru_list_shrink2 [xlabel="N"]
+  fn___bpf_lru_list_shrink2 -> fn___bpf_lru_list_shrink3
+  fn___bpf_lru_list_shrink3 -> local_freelist_check2
+  local_freelist_check2 -> local_freelist_check4 [xlabel = "Y"]
+  local_freelist_check2 -> common_lru_check2 [xlabel = "N"]
+  common_lru_check2 -> local_freelist_check5 [xlabel = "Y"]
+  common_lru_check2 -> fn_htab_lru_map_update_elem_ENOMEM [xlabel = "N"]
+  local_freelist_check5 -> fn_htab_lru_map_update_elem [xlabel = "Y"]
+  local_freelist_check5 -> local_freelist_check6 [xlabel = "N"]
+  local_freelist_check6 -> local_freelist_check7
+  local_freelist_check7 -> fn_htab_lru_map_update_elem
+
+  fn_htab_lru_map_update_elem -> fn_htab_lru_map_update_elem3 [xlabel = "Y"]
+  fn_htab_lru_map_update_elem -> fn_htab_lru_map_update_elem2  [xlabel = "N"]
+  fn_htab_lru_map_update_elem2 ->
+    fn_htab_lru_map_update_elem_ENOMEM [xlabel = "Y"]
+  fn_htab_lru_map_update_elem2 -> local_freelist_check5 [xlabel = "N"]
+  fn_htab_lru_map_update_elem3 -> fn_htab_lru_map_update_elem4
+
+  use_local_node -> fn_htab_lru_map_update_elem4
+  fn_bpf_lru_list_pop_free_to_local2 -> fn_htab_lru_map_update_elem4
+  local_freelist_check4 -> fn_htab_lru_map_update_elem4
+
+  fn_htab_lru_map_update_elem4 -> fn_htab_lru_map_update_elem5 [headlabel="Success"]
+  fn_htab_lru_map_update_elem4 ->
+    fn_htab_lru_map_update_elem_EBUSY [xlabel="Hashtab lock failed"]
+  fn_htab_lru_map_update_elem4 ->
+    fn_htab_lru_map_update_elem_EEXIST [xlabel="BPF_EXIST set and\nkey already exists"]
+  fn_htab_lru_map_update_elem4 ->
+    fn_htab_lru_map_update_elem_ENOENT [headlabel="BPF_NOEXIST set\nand no such entry"]
+
+  // Create invisible pad nodes to line up various nodes
+  pad0 [style=invis]
+  pad1 [style=invis]
+  pad2 [style=invis]
+  pad3 [style=invis]
+  pad4 [style=invis]
+
+  // Line up the key with the top of the graph
+  no_lock -> local_lock [style=invis]
+  local_lock -> lru_lock [style=invis]
+  lru_lock -> hash_lock [style=invis]
+  hash_lock -> remote_lock [style=invis]
+  remote_lock -> local_freelist_check5 [style=invis]
+  remote_lock -> fn___bpf_lru_list_shrink [style=invis]
+
+  // Line up return code nodes at the bottom of the graph
+  fn_htab_lru_map_update_elem -> pad0 [style=invis]
+  pad0 -> pad1 [style=invis]
+  pad1 -> pad2 [style=invis]
+  //pad2-> fn_htab_lru_map_update_elem_ENOMEM [style=invis]
+  fn_htab_lru_map_update_elem4 -> pad3 [style=invis]
+  pad3 -> fn_htab_lru_map_update_elem5  [style=invis]
+  pad3 -> fn_htab_lru_map_update_elem_EBUSY  [style=invis]
+  pad3 -> fn_htab_lru_map_update_elem_EEXIST  [style=invis]
+  pad3 -> fn_htab_lru_map_update_elem_ENOENT  [style=invis]
+
+  // Reduce diagram width by forcing some nodes to appear above others
+  local_freelist_check4 -> fn_htab_lru_map_update_elem3 [style=invis]
+  common_lru_check2 -> pad4 [style=invis]
+  pad4 -> local_freelist_check5 [style=invis]
+}