1 .. SPDX-License-Identifier: GPL-2.0
2
3 =============
4 False Sharing
5 =============
6
7 What is False Sharing
8 =====================
9 False sharing is related with cache mechanism of maintaining the data
10 coherence of one cache line stored in multiple CPU's caches; then
11 academic definition for it is in [1]_. Consider a struct with a
12 refcount and a string::
13
14 struct foo {
15 refcount_t refcount;
16 ...
17 char name[16];
18 } ____cacheline_internodealigned_in_smp;
19
20 Member 'refcount'(A) and 'name'(B) _share_ one cache line like below::
21
22 +-----------+ +-----------+
23 | CPU 0 | | CPU 1 |
24 +-----------+ +-----------+
25 / |
26 / |
27 V V
28 +----------------------+ +----------------------+
29 | A B | Cache 0 | A B | Cache 1
30 +----------------------+ +----------------------+
31 | |
32 ---------------------------+------------------+-----------------------------
33 | |
34 +----------------------+
35 | |
36 +----------------------+
37 Main Memory | A B |
38 +----------------------+
39
40 'refcount' is modified frequently, but 'name' is set once at object
41 creation time and is never modified. When many CPUs access 'foo' at
42 the same time, with 'refcount' being only bumped by one CPU frequently
43 and 'name' being read by other CPUs, all those reading CPUs have to
44 reload the whole cache line over and over due to the 'sharing', even
45 though 'name' is never changed.
46
47 There are many real-world cases of performance regressions caused by
48 false sharing. One of these is a rw_semaphore 'mmap_lock' inside
49 mm_struct struct, whose cache line layout change triggered a
50 regression and Linus analyzed in [2]_.
51
52 There are two key factors for a harmful false sharing:
53
54 * A global datum accessed (shared) by many CPUs
55 * In the concurrent accesses to the data, there is at least one write
56 operation: write/write or write/read cases.
57
58 The sharing could be from totally unrelated kernel components, or
59 different code paths of the same kernel component.
60
61
62 False Sharing Pitfalls
63 ======================
64 Back in time when one platform had only one or a few CPUs, hot data
65 members could be purposely put in the same cache line to make them
66 cache hot and save cacheline/TLB, like a lock and the data protected
67 by it. But for recent large system with hundreds of CPUs, this may
68 not work when the lock is heavily contended, as the lock owner CPU
69 could write to the data, while other CPUs are busy spinning the lock.
70
71 Looking at past cases, there are several frequently occurring patterns
72 for false sharing:
73
74 * lock (spinlock/mutex/semaphore) and data protected by it are
75 purposely put in one cache line.
76 * global data being put together in one cache line. Some kernel
77 subsystems have many global parameters of small size (4 bytes),
78 which can easily be grouped together and put into one cache line.
79 * data members of a big data structure randomly sitting together
80 without being noticed (cache line is usually 64 bytes or more),
81 like 'mem_cgroup' struct.
82
83 Following 'mitigation' section provides real-world examples.
84
85 False sharing could easily happen unless they are intentionally
86 checked, and it is valuable to run specific tools for performance
87 critical workloads to detect false sharing affecting performance case
88 and optimize accordingly.
89
90
91 How to detect and analyze False Sharing
92 ========================================
93 perf record/report/stat are widely used for performance tuning, and
94 once hotspots are detected, tools like 'perf-c2c' and 'pahole' can
95 be further used to detect and pinpoint the possible false sharing
96 data structures. 'addr2line' is also good at decoding instruction
97 pointer when there are multiple layers of inline functions.
98
99 perf-c2c can capture the cache lines with most false sharing hits,
100 decoded functions (line number of file) accessing that cache line,
101 and in-line offset of the data. Simple commands are::
102
103 $ perf c2c record -ag sleep 3
104 $ perf c2c report --call-graph none -k vmlinux
105
106 When running above during testing will-it-scale's tlb_flush1 case,
107 perf reports something like::
108
109 Total records : 1658231
110 Locked Load/Store Operations : 89439
111 Load Operations : 623219
112 Load Local HITM : 92117
113 Load Remote HITM : 139
114
115 #----------------------------------------------------------------------
116 4 0 2374 0 0 0 0xff1100088366d880
117 #----------------------------------------------------------------------
118 0.00% 42.29% 0.00% 0.00% 0.00% 0x8 1 1 0xffffffff81373b7b 0 231 129 5312 64 [k] __mod_lruvec_page_state [kernel.vmlinux] memcontrol.h:752 1
119 0.00% 13.10% 0.00% 0.00% 0.00% 0x8 1 1 0xffffffff81374718 0 226 97 3551 64 [k] folio_lruvec_lock_irqsave [kernel.vmlinux] memcontrol.h:752 1
120 0.00% 11.20% 0.00% 0.00% 0.00% 0x8 1 1 0xffffffff812c29bf 0 170 136 555 64 [k] lru_add_fn [kernel.vmlinux] mm_inline.h:41 1
121 0.00% 7.62% 0.00% 0.00% 0.00% 0x8 1 1 0xffffffff812c3ec5 0 175 108 632 64 [k] release_pages [kernel.vmlinux] mm_inline.h:41 1
122 0.00% 23.29% 0.00% 0.00% 0.00% 0x10 1 1 0xffffffff81372d0a 0 234 279 1051 64 [k] __mod_memcg_lruvec_state [kernel.vmlinux] memcontrol.c:736 1
123
124 A nice introduction for perf-c2c is [3]_.
125
126 'pahole' decodes data structure layouts delimited in cache line
127 granularity. Users can match the offset in perf-c2c output with
128 pahole's decoding to locate the exact data members. For global
129 data, users can search the data address in System.map.
130
131
132 Possible Mitigations
133 ====================
134 False sharing does not always need to be mitigated. False sharing
135 mitigations should balance performance gains with complexity and
136 space consumption. Sometimes, lower performance is OK, and it's
137 unnecessary to hyper-optimize every rarely used data structure or
138 a cold data path.
139
140 False sharing hurting performance cases are seen more frequently with
141 core count increasing. Because of these detrimental effects, many
142 patches have been proposed across variety of subsystems (like
143 networking and memory management) and merged. Some common mitigations
144 (with examples) are:
145
146 * Separate hot global data in its own dedicated cache line, even if it
147 is just a 'short' type. The downside is more consumption of memory,
148 cache line and TLB entries.
149
150 - Commit 91b6d3256356 ("net: cache align tcp_memory_allocated, tcp_sockets_allocated")
151
152 * Reorganize the data structure, separate the interfering members to
153 different cache lines. One downside is it may introduce new false
154 sharing of other members.
155
156 - Commit 802f1d522d5f ("mm: page_counter: re-layout structure to reduce false sharing")
157
158 * Replace 'write' with 'read' when possible, especially in loops.
159 Like for some global variable, use compare(read)-then-write instead
160 of unconditional write. For example, use::
161
162 if (!test_bit(XXX))
163 set_bit(XXX);
164
165 instead of directly "set_bit(XXX);", similarly for atomic_t data::
166
167 if (atomic_read(XXX) == AAA)
168 atomic_set(XXX, BBB);
169
170 - Commit 7b1002f7cfe5 ("bcache: fixup bcache_dev_sectors_dirty_add() multithreaded CPU false sharing")
171 - Commit 292648ac5cf1 ("mm: gup: allow FOLL_PIN to scale in SMP")
172
173 * Turn hot global data to 'per-cpu data + global data' when possible,
174 or reasonably increase the threshold for syncing per-cpu data to
175 global data, to reduce or postpone the 'write' to that global data.
176
177 - Commit 520f897a3554 ("ext4: use percpu_counters for extent_status cache hits/misses")
178 - Commit 56f3547bfa4d ("mm: adjust vm_committed_as_batch according to vm overcommit policy")
179
180 Surely, all mitigations should be carefully verified to not cause side
181 effects. To avoid introducing false sharing when coding, it's better
182 to:
183
184 * Be aware of cache line boundaries
185 * Group mostly read-only fields together
186 * Group things that are written at the same time together
187 * Separate frequently read and frequently written fields on
188 different cache lines.
189
190 and better add a comment stating the false sharing consideration.
191
192 One note is, sometimes even after a severe false sharing is detected
193 and solved, the performance may still have no obvious improvement as
194 the hotspot switches to a new place.
195
196
197 Miscellaneous
198 =============
199 One open issue is that kernel has an optional data structure
200 randomization mechanism, which also randomizes the situation of cache
201 line sharing of data members.
202
203
204 .. [1] https://en.wikipedia.org/wiki/False_sharing
205 .. [2] https://lore.kernel.org/lkml/CAHk-=whoqV=cX5VC80mmR9rr+Z+yQ6fiQZm36Fb-izsanHg23w@mail.gmail.com/
206 .. [3] https://joemario.github.io/blog/2016/09/01/c2c-blog/
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