1
2 .. _local_ops:
3
4 =================================================
5 Semantics and Behavior of Local Atomic Operations
6 =================================================
7
8 :Author: Mathieu Desnoyers
9
10
11 This document explains the purpose of the local atomic operations, how
12 to implement them for any given architecture and shows how they can be used
13 properly. It also stresses on the precautions that must be taken when reading
14 those local variables across CPUs when the order of memory writes matters.
15
16 .. note::
17
18 Note that ``local_t`` based operations are not recommended for general
19 kernel use. Please use the ``this_cpu`` operations instead unless there is
20 really a special purpose. Most uses of ``local_t`` in the kernel have been
21 replaced by ``this_cpu`` operations. ``this_cpu`` operations combine the
22 relocation with the ``local_t`` like semantics in a single instruction and
23 yield more compact and faster executing code.
24
25
26 Purpose of local atomic operations
27 ==================================
28
29 Local atomic operations are meant to provide fast and highly reentrant per CPU
30 counters. They minimize the performance cost of standard atomic operations by
31 removing the LOCK prefix and memory barriers normally required to synchronize
32 across CPUs.
33
34 Having fast per CPU atomic counters is interesting in many cases: it does not
35 require disabling interrupts to protect from interrupt handlers and it permits
36 coherent counters in NMI handlers. It is especially useful for tracing purposes
37 and for various performance monitoring counters.
38
39 Local atomic operations only guarantee variable modification atomicity wrt the
40 CPU which owns the data. Therefore, care must taken to make sure that only one
41 CPU writes to the ``local_t`` data. This is done by using per cpu data and
42 making sure that we modify it from within a preemption safe context. It is
43 however permitted to read ``local_t`` data from any CPU: it will then appear to
44 be written out of order wrt other memory writes by the owner CPU.
45
46
47 Implementation for a given architecture
48 =======================================
49
50 It can be done by slightly modifying the standard atomic operations: only
51 their UP variant must be kept. It typically means removing LOCK prefix (on
52 i386 and x86_64) and any SMP synchronization barrier. If the architecture does
53 not have a different behavior between SMP and UP, including
54 ``asm-generic/local.h`` in your architecture's ``local.h`` is sufficient.
55
56 The ``local_t`` type is defined as an opaque ``signed long`` by embedding an
57 ``atomic_long_t`` inside a structure. This is made so a cast from this type to
58 a ``long`` fails. The definition looks like::
59
60 typedef struct { atomic_long_t a; } local_t;
61
62
63 Rules to follow when using local atomic operations
64 ==================================================
65
66 * Variables touched by local ops must be per cpu variables.
67 * *Only* the CPU owner of these variables must write to them.
68 * This CPU can use local ops from any context (process, irq, softirq, nmi, ...)
69 to update its ``local_t`` variables.
70 * Preemption (or interrupts) must be disabled when using local ops in
71 process context to make sure the process won't be migrated to a
72 different CPU between getting the per-cpu variable and doing the
73 actual local op.
74 * When using local ops in interrupt context, no special care must be
75 taken on a mainline kernel, since they will run on the local CPU with
76 preemption already disabled. I suggest, however, to explicitly
77 disable preemption anyway to make sure it will still work correctly on
78 -rt kernels.
79 * Reading the local cpu variable will provide the current copy of the
80 variable.
81 * Reads of these variables can be done from any CPU, because updates to
82 "``long``", aligned, variables are always atomic. Since no memory
83 synchronization is done by the writer CPU, an outdated copy of the
84 variable can be read when reading some *other* cpu's variables.
85
86
87 How to use local atomic operations
88 ==================================
89
90 ::
91
92 #include <linux/percpu.h>
93 #include <asm/local.h>
94
95 static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
96
97
98 Counting
99 ========
100
101 Counting is done on all the bits of a signed long.
102
103 In preemptible context, use ``get_cpu_var()`` and ``put_cpu_var()`` around
104 local atomic operations: it makes sure that preemption is disabled around write
105 access to the per cpu variable. For instance::
106
107 local_inc(&get_cpu_var(counters));
108 put_cpu_var(counters);
109
110 If you are already in a preemption-safe context, you can use
111 ``this_cpu_ptr()`` instead::
112
113 local_inc(this_cpu_ptr(&counters));
114
115
116
117 Reading the counters
118 ====================
119
120 Those local counters can be read from foreign CPUs to sum the count. Note that
121 the data seen by local_read across CPUs must be considered to be out of order
122 relatively to other memory writes happening on the CPU that owns the data::
123
124 long sum = 0;
125 for_each_online_cpu(cpu)
126 sum += local_read(&per_cpu(counters, cpu));
127
128 If you want to use a remote local_read to synchronize access to a resource
129 between CPUs, explicit ``smp_wmb()`` and ``smp_rmb()`` memory barriers must be used
130 respectively on the writer and the reader CPUs. It would be the case if you use
131 the ``local_t`` variable as a counter of bytes written in a buffer: there should
132 be a ``smp_wmb()`` between the buffer write and the counter increment and also a
133 ``smp_rmb()`` between the counter read and the buffer read.
134
135
136 Here is a sample module which implements a basic per cpu counter using
137 ``local.h``::
138
139 /* test-local.c
140 *
141 * Sample module for local.h usage.
142 */
143
144
145 #include <asm/local.h>
146 #include <linux/module.h>
147 #include <linux/timer.h>
148
149 static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
150
151 static struct timer_list test_timer;
152
153 /* IPI called on each CPU. */
154 static void test_each(void *info)
155 {
156 /* Increment the counter from a non preemptible context */
157 printk("Increment on cpu %d\n", smp_processor_id());
158 local_inc(this_cpu_ptr(&counters));
159
160 /* This is what incrementing the variable would look like within a
161 * preemptible context (it disables preemption) :
162 *
163 * local_inc(&get_cpu_var(counters));
164 * put_cpu_var(counters);
165 */
166 }
167
168 static void do_test_timer(unsigned long data)
169 {
170 int cpu;
171
172 /* Increment the counters */
173 on_each_cpu(test_each, NULL, 1);
174 /* Read all the counters */
175 printk("Counters read from CPU %d\n", smp_processor_id());
176 for_each_online_cpu(cpu) {
177 printk("Read : CPU %d, count %ld\n", cpu,
178 local_read(&per_cpu(counters, cpu)));
179 }
180 mod_timer(&test_timer, jiffies + 1000);
181 }
182
183 static int __init test_init(void)
184 {
185 /* initialize the timer that will increment the counter */
186 timer_setup(&test_timer, do_test_timer, 0);
187 mod_timer(&test_timer, jiffies + 1);
188
189 return 0;
190 }
191
192 static void __exit test_exit(void)
193 {
194 timer_shutdown_sync(&test_timer);
195 }
196
197 module_init(test_init);
198 module_exit(test_exit);
199
200 MODULE_LICENSE("GPL");
201 MODULE_AUTHOR("Mathieu Desnoyers");
202 MODULE_DESCRIPTION("Local Atomic Ops");
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