1.. SPDX-License-Identifier: GPL-2.0
2
3=============
4False Sharing
5=============
6
7What is False Sharing
8=====================
9False sharing is related with cache mechanism of maintaining the data
10coherence of one cache line stored in multiple CPU's caches; then
11academic definition for it is in [1]_. Consider a struct with a
12refcount and a string::
13
14	struct foo {
15		refcount_t refcount;
16		...
17		char name[16];
18	} ____cacheline_internodealigned_in_smp;
19
20Member '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
41creation time and is never modified.  When many CPUs access 'foo' at
42the same time, with 'refcount' being only bumped by one CPU frequently
43and 'name' being read by other CPUs, all those reading CPUs have to
44reload the whole cache line over and over due to the 'sharing', even
45though 'name' is never changed.
46
47There are many real-world cases of performance regressions caused by
48false sharing.  One of these is a rw_semaphore 'mmap_lock' inside
49mm_struct struct, whose cache line layout change triggered a
50regression and Linus analyzed in [2]_.
51
52There 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
58The sharing could be from totally unrelated kernel components, or
59different code paths of the same kernel component.
60
61
62False Sharing Pitfalls
63======================
64Back in time when one platform had only one or a few CPUs, hot data
65members could be purposely put in the same cache line to make them
66cache hot and save cacheline/TLB, like a lock and the data protected
67by it.  But for recent large system with hundreds of CPUs, this may
68not work when the lock is heavily contended, as the lock owner CPU
69could write to the data, while other CPUs are busy spinning the lock.
70
71Looking at past cases, there are several frequently occurring patterns
72for 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
83Following 'mitigation' section provides real-world examples.
84
85False sharing could easily happen unless they are intentionally
86checked, and it is valuable to run specific tools for performance
87critical workloads to detect false sharing affecting performance case
88and optimize accordingly.
89
90
91How to detect and analyze False Sharing
92========================================
93perf record/report/stat are widely used for performance tuning, and
94once hotspots are detected, tools like 'perf-c2c' and 'pahole' can
95be further used to detect and pinpoint the possible false sharing
96data structures.  'addr2line' is also good at decoding instruction
97pointer when there are multiple layers of inline functions.
98
99perf-c2c can capture the cache lines with most false sharing hits,
100decoded functions (line number of file) accessing that cache line,
101and 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
106When running above during testing will-it-scale's tlb_flush1 case,
107perf 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
124A nice introduction for perf-c2c is [3]_.
125
126'pahole' decodes data structure layouts delimited in cache line
127granularity.  Users can match the offset in perf-c2c output with
128pahole's decoding to locate the exact data members.  For global
129data, users can search the data address in System.map.
130
131
132Possible Mitigations
133====================
134False sharing does not always need to be mitigated.  False sharing
135mitigations should balance performance gains with complexity and
136space consumption.  Sometimes, lower performance is OK, and it's
137unnecessary to hyper-optimize every rarely used data structure or
138a cold data path.
139
140False sharing hurting performance cases are seen more frequently with
141core count increasing.  Because of these detrimental effects, many
142patches have been proposed across variety of subsystems (like
143networking 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
180Surely, all mitigations should be carefully verified to not cause side
181effects.  To avoid introducing false sharing when coding, it's better
182to:
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
190and better add a comment stating the false sharing consideration.
191
192One note is, sometimes even after a severe false sharing is detected
193and solved, the performance may still have no obvious improvement as
194the hotspot switches to a new place.
195
196
197Miscellaneous
198=============
199One open issue is that kernel has an optional data structure
200randomization mechanism, which also randomizes the situation of cache
201line 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/
207