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docs/bpf: Add documentation for new cgroup local storage

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Add some descriptions and examples for BPF_MAP_TYPE_CGRP_STORAGE.
Also illustate the major difference between BPF_MAP_TYPE_CGRP_STORAGE
and BPF_MAP_TYPE_CGROUP_STORAGE and recommend to use
BPF_MAP_TYPE_CGRP_STORAGE instead of BPF_MAP_TYPE_CGROUP_STORAGE
in the end.

Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221026042922.676383-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Yonghong Song 2022-10-25 21:29:22 -07:00 • committed by Alexei Starovoitov
parent 0a1b69d1c7
commit d43198017e
1 changed files with 109 additions and 0 deletions
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Documentation/bpf/map_cgrp_storage.rst Normal file
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.. SPDX-License-Identifier: GPL-2.0-only
.. Copyright (C) 2022 Meta Platforms, Inc. and affiliates.
=========================
BPF_MAP_TYPE_CGRP_STORAGE
=========================
The ``BPF_MAP_TYPE_CGRP_STORAGE`` map type represents a local fix-sized
storage for cgroups. It is only available with ``CONFIG_CGROUPS``.
The programs are made available by the same Kconfig. The
data for a particular cgroup can be retrieved by looking up the map
with that cgroup.
This document describes the usage and semantics of the
``BPF_MAP_TYPE_CGRP_STORAGE`` map type.
Usage
=====
The map key must be ``sizeof(int)`` representing a cgroup fd.
To access the storage in a program, use ``bpf_cgrp_storage_get``::
void *bpf_cgrp_storage_get(struct bpf_map *map, struct cgroup *cgroup, void *value, u64 flags)
``flags`` could be 0 or ``BPF_LOCAL_STORAGE_GET_F_CREATE`` which indicates that
a new local storage will be created if one does not exist.
The local storage can be removed with ``bpf_cgrp_storage_delete``::
long bpf_cgrp_storage_delete(struct bpf_map *map, struct cgroup *cgroup)
The map is available to all program types.
Examples
========
A BPF program example with BPF_MAP_TYPE_CGRP_STORAGE::
#include <vmlinux.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>
struct {
__uint(type, BPF_MAP_TYPE_CGRP_STORAGE);
__uint(map_flags, BPF_F_NO_PREALLOC);
__type(key, int);
__type(value, long);
} cgrp_storage SEC(".maps");
SEC("tp_btf/sys_enter")
int BPF_PROG(on_enter, struct pt_regs *regs, long id)
{
struct task_struct *task = bpf_get_current_task_btf();
long *ptr;
ptr = bpf_cgrp_storage_get(&cgrp_storage, task->cgroups->dfl_cgrp, 0,
BPF_LOCAL_STORAGE_GET_F_CREATE);
if (ptr)
__sync_fetch_and_add(ptr, 1);
return 0;
}
Userspace accessing map declared above::
#include <linux/bpf.h>
#include <linux/libbpf.h>
__u32 map_lookup(struct bpf_map *map, int cgrp_fd)
{
__u32 *value;
value = bpf_map_lookup_elem(bpf_map__fd(map), &cgrp_fd);
if (value)
return *value;
return 0;
}
Difference Between BPF_MAP_TYPE_CGRP_STORAGE and BPF_MAP_TYPE_CGROUP_STORAGE
============================================================================
The old cgroup storage map ``BPF_MAP_TYPE_CGROUP_STORAGE`` has been marked as
deprecated (renamed to ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``). The new
``BPF_MAP_TYPE_CGRP_STORAGE`` map should be used instead. The following
illusates the main difference between ``BPF_MAP_TYPE_CGRP_STORAGE`` and
``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``.
(1). ``BPF_MAP_TYPE_CGRP_STORAGE`` can be used by all program types while
``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` is available only to cgroup program types
like BPF_CGROUP_INET_INGRESS or BPF_CGROUP_SOCK_OPS, etc.
(2). ``BPF_MAP_TYPE_CGRP_STORAGE`` supports local storage for more than one
cgroup while ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` only supports one cgroup
which is attached by a BPF program.
(3). ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` allocates local storage at attach time so
``bpf_get_local_storage()`` always returns non-NULL local storage.
``BPF_MAP_TYPE_CGRP_STORAGE`` allocates local storage at runtime so
it is possible that ``bpf_cgrp_storage_get()`` may return null local storage.
To avoid such null local storage issue, user space can do
``bpf_map_update_elem()`` to pre-allocate local storage before a BPF program
is attached.
(4). ``BPF_MAP_TYPE_CGRP_STORAGE`` supports deleting local storage by a BPF program
while ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` only deletes storage during
prog detach time.
So overall, ``BPF_MAP_TYPE_CGRP_STORAGE`` supports all ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``
functionality and beyond. It is recommended to use ``BPF_MAP_TYPE_CGRP_STORAGE``
instead of ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``.