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1 | Semantics and Behavior of Local Atomic Operations |
2 | ||
3 | Mathieu Desnoyers | |
4 | ||
5 | ||
6 | This document explains the purpose of the local atomic operations, how | |
7 | to implement them for any given architecture and shows how they can be used | |
8 | properly. It also stresses on the precautions that must be taken when reading | |
9 | those local variables across CPUs when the order of memory writes matters. | |
10 | ||
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11 | Note that local_t based operations are not recommended for general kernel use. |
12 | Please use the this_cpu operations instead unless there is really a special purpose. | |
13 | Most uses of local_t in the kernel have been replaced by this_cpu operations. | |
14 | this_cpu operations combine the relocation with the local_t like semantics in | |
15 | a single instruction and yield more compact and faster executing code. | |
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16 | |
17 | ||
18 | * Purpose of local atomic operations | |
19 | ||
20 | Local atomic operations are meant to provide fast and highly reentrant per CPU | |
21 | counters. They minimize the performance cost of standard atomic operations by | |
22 | removing the LOCK prefix and memory barriers normally required to synchronize | |
23 | across CPUs. | |
24 | ||
25 | Having fast per CPU atomic counters is interesting in many cases : it does not | |
26 | require disabling interrupts to protect from interrupt handlers and it permits | |
27 | coherent counters in NMI handlers. It is especially useful for tracing purposes | |
28 | and for various performance monitoring counters. | |
29 | ||
30 | Local atomic operations only guarantee variable modification atomicity wrt the | |
31 | CPU which owns the data. Therefore, care must taken to make sure that only one | |
32 | CPU writes to the local_t data. This is done by using per cpu data and making | |
33 | sure that we modify it from within a preemption safe context. It is however | |
34 | permitted to read local_t data from any CPU : it will then appear to be written | |
0e1ccb96 | 35 | out of order wrt other memory writes by the owner CPU. |
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36 | |
37 | ||
38 | * Implementation for a given architecture | |
39 | ||
40 | It can be done by slightly modifying the standard atomic operations : only | |
41 | their UP variant must be kept. It typically means removing LOCK prefix (on | |
19f59460 | 42 | i386 and x86_64) and any SMP synchronization barrier. If the architecture does |
f1f8810c | 43 | not have a different behavior between SMP and UP, including asm-generic/local.h |
d9195881 | 44 | in your architecture's local.h is sufficient. |
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45 | |
46 | The local_t type is defined as an opaque signed long by embedding an | |
47 | atomic_long_t inside a structure. This is made so a cast from this type to a | |
48 | long fails. The definition looks like : | |
49 | ||
50 | typedef struct { atomic_long_t a; } local_t; | |
51 | ||
52 | ||
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53 | * Rules to follow when using local atomic operations |
54 | ||
55 | - Variables touched by local ops must be per cpu variables. | |
56 | - _Only_ the CPU owner of these variables must write to them. | |
57 | - This CPU can use local ops from any context (process, irq, softirq, nmi, ...) | |
58 | to update its local_t variables. | |
59 | - Preemption (or interrupts) must be disabled when using local ops in | |
60 | process context to make sure the process won't be migrated to a | |
61 | different CPU between getting the per-cpu variable and doing the | |
62 | actual local op. | |
63 | - When using local ops in interrupt context, no special care must be | |
64 | taken on a mainline kernel, since they will run on the local CPU with | |
65 | preemption already disabled. I suggest, however, to explicitly | |
66 | disable preemption anyway to make sure it will still work correctly on | |
67 | -rt kernels. | |
68 | - Reading the local cpu variable will provide the current copy of the | |
69 | variable. | |
70 | - Reads of these variables can be done from any CPU, because updates to | |
71 | "long", aligned, variables are always atomic. Since no memory | |
72 | synchronization is done by the writer CPU, an outdated copy of the | |
73 | variable can be read when reading some _other_ cpu's variables. | |
74 | ||
75 | ||
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76 | * How to use local atomic operations |
77 | ||
78 | #include <linux/percpu.h> | |
79 | #include <asm/local.h> | |
80 | ||
81 | static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0); | |
82 | ||
83 | ||
84 | * Counting | |
85 | ||
86 | Counting is done on all the bits of a signed long. | |
87 | ||
88 | In preemptible context, use get_cpu_var() and put_cpu_var() around local atomic | |
89 | operations : it makes sure that preemption is disabled around write access to | |
90 | the per cpu variable. For instance : | |
91 | ||
92 | local_inc(&get_cpu_var(counters)); | |
93 | put_cpu_var(counters); | |
94 | ||
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95 | If you are already in a preemption-safe context, you can use |
96 | this_cpu_ptr() instead. | |
f1f8810c | 97 | |
7d94a82e | 98 | local_inc(this_cpu_ptr(&counters)); |
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99 | |
100 | ||
101 | ||
102 | * Reading the counters | |
103 | ||
104 | Those local counters can be read from foreign CPUs to sum the count. Note that | |
105 | the data seen by local_read across CPUs must be considered to be out of order | |
106 | relatively to other memory writes happening on the CPU that owns the data. | |
107 | ||
108 | long sum = 0; | |
109 | for_each_online_cpu(cpu) | |
110 | sum += local_read(&per_cpu(counters, cpu)); | |
111 | ||
112 | If you want to use a remote local_read to synchronize access to a resource | |
113 | between CPUs, explicit smp_wmb() and smp_rmb() memory barriers must be used | |
114 | respectively on the writer and the reader CPUs. It would be the case if you use | |
115 | the local_t variable as a counter of bytes written in a buffer : there should | |
116 | be a smp_wmb() between the buffer write and the counter increment and also a | |
117 | smp_rmb() between the counter read and the buffer read. | |
118 | ||
119 | ||
120 | Here is a sample module which implements a basic per cpu counter using local.h. | |
121 | ||
122 | --- BEGIN --- | |
123 | /* test-local.c | |
124 | * | |
125 | * Sample module for local.h usage. | |
126 | */ | |
127 | ||
128 | ||
129 | #include <asm/local.h> | |
130 | #include <linux/module.h> | |
131 | #include <linux/timer.h> | |
132 | ||
133 | static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0); | |
134 | ||
135 | static struct timer_list test_timer; | |
136 | ||
137 | /* IPI called on each CPU. */ | |
138 | static void test_each(void *info) | |
139 | { | |
140 | /* Increment the counter from a non preemptible context */ | |
141 | printk("Increment on cpu %d\n", smp_processor_id()); | |
7d94a82e | 142 | local_inc(this_cpu_ptr(&counters)); |
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143 | |
144 | /* This is what incrementing the variable would look like within a | |
145 | * preemptible context (it disables preemption) : | |
146 | * | |
147 | * local_inc(&get_cpu_var(counters)); | |
148 | * put_cpu_var(counters); | |
149 | */ | |
150 | } | |
151 | ||
152 | static void do_test_timer(unsigned long data) | |
153 | { | |
154 | int cpu; | |
155 | ||
156 | /* Increment the counters */ | |
02d43b1d | 157 | on_each_cpu(test_each, NULL, 1); |
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158 | /* Read all the counters */ |
159 | printk("Counters read from CPU %d\n", smp_processor_id()); | |
160 | for_each_online_cpu(cpu) { | |
161 | printk("Read : CPU %d, count %ld\n", cpu, | |
162 | local_read(&per_cpu(counters, cpu))); | |
163 | } | |
164 | del_timer(&test_timer); | |
165 | test_timer.expires = jiffies + 1000; | |
166 | add_timer(&test_timer); | |
167 | } | |
168 | ||
169 | static int __init test_init(void) | |
170 | { | |
171 | /* initialize the timer that will increment the counter */ | |
172 | init_timer(&test_timer); | |
173 | test_timer.function = do_test_timer; | |
174 | test_timer.expires = jiffies + 1; | |
175 | add_timer(&test_timer); | |
176 | ||
177 | return 0; | |
178 | } | |
179 | ||
180 | static void __exit test_exit(void) | |
181 | { | |
182 | del_timer_sync(&test_timer); | |
183 | } | |
184 | ||
185 | module_init(test_init); | |
186 | module_exit(test_exit); | |
187 | ||
188 | MODULE_LICENSE("GPL"); | |
189 | MODULE_AUTHOR("Mathieu Desnoyers"); | |
190 | MODULE_DESCRIPTION("Local Atomic Ops"); | |
191 | --- END --- |