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diff --git a/Documentation/local_ops.txt b/Documentation/local_ops.txt new file mode 100644 index 000000000000..b0aca0705d1e --- /dev/null +++ b/Documentation/local_ops.txt @@ -0,0 +1,163 @@ + 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 on 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 sychronization barrier. If the architecture does +not have a different behavior between SMP and UP, including asm-generic/local.h +in your archtecture'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; + + +* 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, 0, 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 --- |