Documentation/core-api/local_ops.rst

   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             del_timer_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");