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- Semantics and Behavior of Local Atomic Operations
-
- Mathieu Desnoyers
-
-
- This document explains the purpose of the local atomic operations, how
-to implement them for any given architecture and shows how they can be used
-properly. It also stresses on the precautions that must be taken when reading
-those local variables across CPUs when the order of memory writes matters.
-
-
-
-* Purpose of local atomic operations
-
-Local atomic operations are meant to provide fast and highly reentrant per CPU
-counters. They minimize the performance cost of standard atomic operations by
-removing the LOCK prefix and memory barriers normally required to synchronize
-across CPUs.
-
-Having fast per CPU atomic counters is interesting in many cases : it does not
-require disabling interrupts to protect from interrupt handlers and it permits
-coherent counters in NMI handlers. It is especially useful for tracing purposes
-and for various performance monitoring counters.
-
-Local atomic operations only guarantee variable modification atomicity wrt the
-CPU which owns the data. Therefore, care must taken to make sure that only one
-CPU writes to the local_t data. This is done by using per cpu data and making
-sure that we modify it from within a preemption safe context. It is however
-permitted to read local_t data from any CPU : it will then appear to be written
-out of order wrt other memory writes by the owner CPU.
-
-
-* Implementation for a given architecture
-
-It can be done by slightly modifying the standard atomic operations : only
-their UP variant must be kept. It typically means removing LOCK prefix (on
-i386 and x86_64) and any SMP synchronization barrier. If the architecture does
-not have a different behavior between SMP and UP, including asm-generic/local.h
-in your architecture's local.h is sufficient.
-
-The local_t type is defined as an opaque signed long by embedding an
-atomic_long_t inside a structure. This is made so a cast from this type to a
-long fails. The definition looks like :
-
-typedef struct { atomic_long_t a; } local_t;
-
-
-* Rules to follow when using local atomic operations
-
-- Variables touched by local ops must be per cpu variables.
-- _Only_ the CPU owner of these variables must write to them.
-- This CPU can use local ops from any context (process, irq, softirq, nmi, ...)
- to update its local_t variables.
-- Preemption (or interrupts) must be disabled when using local ops in
- process context to make sure the process won't be migrated to a
- different CPU between getting the per-cpu variable and doing the
- actual local op.
-- When using local ops in interrupt context, no special care must be
- taken on a mainline kernel, since they will run on the local CPU with
- preemption already disabled. I suggest, however, to explicitly
- disable preemption anyway to make sure it will still work correctly on
- -rt kernels.
-- Reading the local cpu variable will provide the current copy of the
- variable.
-- Reads of these variables can be done from any CPU, because updates to
- "long", aligned, variables are always atomic. Since no memory
- synchronization is done by the writer CPU, an outdated copy of the
- variable can be read when reading some _other_ cpu's variables.
-
-
-* How to use local atomic operations
-
-#include <linux/percpu.h>
-#include <asm/local.h>
-
-static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
-
-
-* Counting
-
-Counting is done on all the bits of a signed long.
-
-In preemptible context, use get_cpu_var() and put_cpu_var() around local atomic
-operations : it makes sure that preemption is disabled around write access to
-the per cpu variable. For instance :
-
- local_inc(&get_cpu_var(counters));
- put_cpu_var(counters);
-
-If you are already in a preemption-safe context, you can directly use
-__get_cpu_var() instead.
-
- local_inc(&__get_cpu_var(counters));
-
-
-
-* Reading the counters
-
-Those local counters can be read from foreign CPUs to sum the count. Note that
-the data seen by local_read across CPUs must be considered to be out of order
-relatively to other memory writes happening on the CPU that owns the data.
-
- long sum = 0;
- for_each_online_cpu(cpu)
- sum += local_read(&per_cpu(counters, cpu));
-
-If you want to use a remote local_read to synchronize access to a resource
-between CPUs, explicit smp_wmb() and smp_rmb() memory barriers must be used
-respectively on the writer and the reader CPUs. It would be the case if you use
-the local_t variable as a counter of bytes written in a buffer : there should
-be a smp_wmb() between the buffer write and the counter increment and also a
-smp_rmb() between the counter read and the buffer read.
-
-
-Here is a sample module which implements a basic per cpu counter using local.h.
-
---- BEGIN ---
-/* test-local.c
- *
- * Sample module for local.h usage.
- */
-
-
-#include <asm/local.h>
-#include <linux/module.h>
-#include <linux/timer.h>
-
-static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
-
-static struct timer_list test_timer;
-
-/* IPI called on each CPU. */
-static void test_each(void *info)
-{
- /* Increment the counter from a non preemptible context */
- printk("Increment on cpu %d\n", smp_processor_id());
- local_inc(&__get_cpu_var(counters));
-
- /* This is what incrementing the variable would look like within a
- * preemptible context (it disables preemption) :
- *
- * local_inc(&get_cpu_var(counters));
- * put_cpu_var(counters);
- */
-}
-
-static void do_test_timer(unsigned long data)
-{
- int cpu;
-
- /* Increment the counters */
- on_each_cpu(test_each, NULL, 1);
- /* Read all the counters */
- printk("Counters read from CPU %d\n", smp_processor_id());
- for_each_online_cpu(cpu) {
- printk("Read : CPU %d, count %ld\n", cpu,
- local_read(&per_cpu(counters, cpu)));
- }
- del_timer(&test_timer);
- test_timer.expires = jiffies + 1000;
- add_timer(&test_timer);
-}
-
-static int __init test_init(void)
-{
- /* initialize the timer that will increment the counter */
- init_timer(&test_timer);
- test_timer.function = do_test_timer;
- test_timer.expires = jiffies + 1;
- add_timer(&test_timer);
-
- return 0;
-}
-
-static void __exit test_exit(void)
-{
- del_timer_sync(&test_timer);
-}
-
-module_init(test_init);
-module_exit(test_exit);
-
-MODULE_LICENSE("GPL");
-MODULE_AUTHOR("Mathieu Desnoyers");
-MODULE_DESCRIPTION("Local Atomic Ops");
---- END ---