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