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@xxxxxxxxxxxxx> --- 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 | 62 ++++++++ Documentation/bpf/map_lru_hash_update.dot | 166 ++++++++++++++++++++++ 2 files changed, 228 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 8669426264c6..61602ce26561 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 @@ -206,3 +207,64 @@ 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 +-------------------------------------- + +An LRU hashmap type consists of two properties: Firstly, it is a hash map and +hence is indexable by key for constant time lookups. Secondly, when at map +capacity, map updates will trigger eviction of old entries based on the age of +the elements in a set of lists. Each of these properties may be either global +or per-CPU, depending on the map type and flags used to create the map: + ++------------------------+---------------------------+----------------------------------+ +| | ``BPF_MAP_TYPE_LRU_HASH`` | ``BPF_MAP_TYPE_LRU_PERCPU_HASH`` | ++========================+===========================+==================================+ +| ``BPF_NO_COMMON_LRU`` | Per-CPU LRU, global map | Per-CPU LRU, per-cpu map | ++------------------------+---------------------------+----------------------------------+ +| ``!BPF_NO_COMMON_LRU`` | Global LRU, global map | Global LRU, per-cpu map | ++------------------------+---------------------------+----------------------------------+ + +Notably, 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 + +Even if an LRU node may be acquired, maps of type ``BPF_MAP_TYPE_LRU_HASH`` +may fail to insert the entry into the map if other CPUs are heavily contending +on the global hashmap lock. + +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 + +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_NO_COMMON_LRU`` then all map +properties would be per-cpu. + +The dot file source for the above figure uses internal kernel function names +for the node names in order to make the corresponding logic easier to find. diff --git a/Documentation/bpf/map_lru_hash_update.dot b/Documentation/bpf/map_lru_hash_update.dot new file mode 100644 index 000000000000..3a44ebec501e --- /dev/null +++ b/Documentation/bpf/map_lru_hash_update.dot @@ -0,0 +1,166 @@ +// 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. + + // The following corresponds to bpf_lru_pop_free() for common LRU case. + local_freelist_check [shape=diamond,fillcolor=1, + label="Local freelist\nnode available?"]; + // The following corresponds to __local_list_pop_free() for common LRU case. + use_local_node [shape=rectangle, + label="Use node owned\nby this CPU"] + + common_lru_check [shape=diamond, + label="Map created with\ncommon LRU?\n(!BPF_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_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=diamond, + label="Can lock this\nhashtab bucket?"] + fn_htab_lru_map_update_elem5 [shape=rectangle,fillcolor=3, + label="Update hashmap\nwith new element"] + fn_htab_lru_map_update_elem6 [shape=oval,label="return 0"] + fn_htab_lru_map_update_elem_EBUSY [shape=oval,label="return -EBUSY"] + + 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 [xlabel="Y"] + fn_htab_lru_map_update_elem4 -> + fn_htab_lru_map_update_elem_EBUSY [xlabel="N"] + fn_htab_lru_map_update_elem5 -> fn_htab_lru_map_update_elem6 + + // 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_elem_EBUSY [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
