xref: /linux/Documentation/mm/allocation-profiling.rst (revision 7f71507851fc7764b36a3221839607d3a45c2025) 
1.. SPDX-License-Identifier: GPL-2.0
2
3===========================
4MEMORY ALLOCATION PROFILING
5===========================
6
7Low overhead (suitable for production) accounting of all memory allocations,
8tracked by file and line number.
9
10Usage:
11kconfig options:
12- CONFIG_MEM_ALLOC_PROFILING
13
14- CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
15
16- CONFIG_MEM_ALLOC_PROFILING_DEBUG
17  adds warnings for allocations that weren't accounted because of a
18  missing annotation
19
20Boot parameter:
21  sysctl.vm.mem_profiling={0|1|never}[,compressed]
22
23  When set to "never", memory allocation profiling overhead is minimized and it
24  cannot be enabled at runtime (sysctl becomes read-only).
25  When CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=y, default value is "1".
26  When CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=n, default value is "never".
27  "compressed" optional parameter will try to store page tag references in a
28  compact format, avoiding page extensions. This results in improved performance
29  and memory consumption, however it might fail depending on system configuration.
30  If compression fails, a warning is issued and memory allocation profiling gets
31  disabled.
32
33sysctl:
34  /proc/sys/vm/mem_profiling
35
36Runtime info:
37  /proc/allocinfo
38
39Example output::
40
41  root@moria-kvm:~# sort -g /proc/allocinfo|tail|numfmt --to=iec
42        2.8M    22648 fs/kernfs/dir.c:615 func:__kernfs_new_node
43        3.8M      953 mm/memory.c:4214 func:alloc_anon_folio
44        4.0M     1010 drivers/staging/ctagmod/ctagmod.c:20 [ctagmod] func:ctagmod_start
45        4.1M        4 net/netfilter/nf_conntrack_core.c:2567 func:nf_ct_alloc_hashtable
46        6.0M     1532 mm/filemap.c:1919 func:__filemap_get_folio
47        8.8M     2785 kernel/fork.c:307 func:alloc_thread_stack_node
48         13M      234 block/blk-mq.c:3421 func:blk_mq_alloc_rqs
49         14M     3520 mm/mm_init.c:2530 func:alloc_large_system_hash
50         15M     3656 mm/readahead.c:247 func:page_cache_ra_unbounded
51         55M     4887 mm/slub.c:2259 func:alloc_slab_page
52        122M    31168 mm/page_ext.c:270 func:alloc_page_ext
53
54Theory of operation
55===================
56
57Memory allocation profiling builds off of code tagging, which is a library for
58declaring static structs (that typically describe a file and line number in
59some way, hence code tagging) and then finding and operating on them at runtime,
60- i.e. iterating over them to print them in debugfs/procfs.
61
62To add accounting for an allocation call, we replace it with a macro
63invocation, alloc_hooks(), that
64- declares a code tag
65- stashes a pointer to it in task_struct
66- calls the real allocation function
67- and finally, restores the task_struct alloc tag pointer to its previous value.
68
69This allows for alloc_hooks() calls to be nested, with the most recent one
70taking effect. This is important for allocations internal to the mm/ code that
71do not properly belong to the outer allocation context and should be counted
72separately: for example, slab object extension vectors, or when the slab
73allocates pages from the page allocator.
74
75Thus, proper usage requires determining which function in an allocation call
76stack should be tagged. There are many helper functions that essentially wrap
77e.g. kmalloc() and do a little more work, then are called in multiple places;
78we'll generally want the accounting to happen in the callers of these helpers,
79not in the helpers themselves.
80
81To fix up a given helper, for example foo(), do the following:
82- switch its allocation call to the _noprof() version, e.g. kmalloc_noprof()
83
84- rename it to foo_noprof()
85
86- define a macro version of foo() like so:
87
88  #define foo(...) alloc_hooks(foo_noprof(__VA_ARGS__))
89
90It's also possible to stash a pointer to an alloc tag in your own data structures.
91
92Do this when you're implementing a generic data structure that does allocations
93"on behalf of" some other code - for example, the rhashtable code. This way,
94instead of seeing a large line in /proc/allocinfo for rhashtable.c, we can
95break it out by rhashtable type.
96
97To do so:
98- Hook your data structure's init function, like any other allocation function.
99
100- Within your init function, use the convenience macro alloc_tag_record() to
101  record alloc tag in your data structure.
102
103- Then, use the following form for your allocations:
104  alloc_hooks_tag(ht->your_saved_tag, kmalloc_noprof(...))
105