1 Semantics and Behavior of Local Atomic Operations
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.
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13 Most uses of local_t in the kernel have been replaced by this_cpu operations.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.
18 * Purpose of local atomic operations
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
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.
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
35 out of order wrt other memory writes by the owner CPU.
38 * Implementation for a given architecture
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
42 i386 and x86_64) and any SMP synchronization barrier. If the architecture does
43 not have a different behavior between SMP and UP, including asm-generic/local.h
44 in your architecture's local.h is sufficient.
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 :
50 typedef struct { atomic_long_t a; } local_t;
53 * Rules to follow when using local atomic operations
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
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
68 - Reading the local cpu variable will provide the current copy of the
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.
76 * How to use local atomic operations
78 #include <linux/percpu.h>
79 #include <asm/local.h>
81 static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
86 Counting is done on all the bits of a signed long.
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 :
92 local_inc(&get_cpu_var(counters));
93 put_cpu_var(counters);
95 If you are already in a preemption-safe context, you can use
96 this_cpu_ptr() instead.
98 local_inc(this_cpu_ptr(&counters));
102 * Reading the counters
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.
109 for_each_online_cpu(cpu)
110 sum += local_read(&per_cpu(counters, cpu));
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.
120 Here is a sample module which implements a basic per cpu counter using local.h.
125 * Sample module for local.h usage.
129 #include <asm/local.h>
130 #include <linux/module.h>
131 #include <linux/timer.h>
133 static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
135 static struct timer_list test_timer;
137 /* IPI called on each CPU. */
138 static void test_each(void *info)
140 /* Increment the counter from a non preemptible context */
141 printk("Increment on cpu %d\n", smp_processor_id());
142 local_inc(this_cpu_ptr(&counters));
144 /* This is what incrementing the variable would look like within a
145 * preemptible context (it disables preemption) :
147 * local_inc(&get_cpu_var(counters));
148 * put_cpu_var(counters);
152 static void do_test_timer(unsigned long data)
156 /* Increment the counters */
157 on_each_cpu(test_each, NULL, 1);
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)));
164 del_timer(&test_timer);
165 test_timer.expires = jiffies + 1000;
166 add_timer(&test_timer);
169 static int __init test_init(void)
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);
180 static void __exit test_exit(void)
182 del_timer_sync(&test_timer);
185 module_init(test_init);
186 module_exit(test_exit);
188 MODULE_LICENSE("GPL");
189 MODULE_AUTHOR("Mathieu Desnoyers");
190 MODULE_DESCRIPTION("Local Atomic Ops");