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

Joe Stringer
2023-04-01T20:07:14Z

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 <joe@isovalent.com>
---
v4: Move UAPI descriptions outside of the internals section
Fix function reference discrepancies in dot source
Fix incorrect flag references (missing F_)
Simplify logic at bottom of graph for map updates
Add missing return codes to graph for failure cases
v3: Use standard table syntax
Replace inline commit message with reference to commit
Fix incorrect Y/N label for common LRU check
Rename some dotfile variables to reduce confusion between cases
Minor wording touchups
v2: Fix issue that caused initial email submission to fail
---
Documentation/bpf/map_hash.rst | 42 ++++++
Documentation/bpf/map_lru_hash_update.dot | 172 ++++++++++++++++++++++
2 files changed, 214 insertions(+)
create mode 100644 Documentation/bpf/map_lru_hash_update.dot

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.
diff --git a/Documentation/bpf/map_lru_hash_update.dot b/Documentation/bpf/map_lru_hash_update.dot
new file mode 100644
index 000000000000..cbcb0ae806d3
--- /dev/null
+++ b/Documentation/bpf/map_lru_hash_update.dot
@@ -0,0 +1,172 @@
+// 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 [label="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 [label="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]
+}
--
2.34.1