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202 lines
7.2 KiB
202 lines
7.2 KiB
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.. _local_ops: |
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================================================= |
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Semantics and Behavior of Local Atomic Operations |
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================================================= |
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:Author: 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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.. note:: |
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Note that ``local_t`` based operations are not recommended for general |
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kernel use. Please use the ``this_cpu`` operations instead unless there is |
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really a special purpose. Most uses of ``local_t`` in the kernel have been |
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replaced by ``this_cpu`` operations. ``this_cpu`` operations combine the |
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relocation with the ``local_t`` like semantics in a single instruction and |
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yield more compact and faster executing code. |
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Purpose of local atomic operations |
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================================== |
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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 |
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making sure that we modify it from within a preemption safe context. It is |
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however permitted to read ``local_t`` data from any CPU: it will then appear to |
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be written out of order wrt other memory writes by the owner CPU. |
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Implementation for a given architecture |
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======================================= |
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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 |
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``asm-generic/local.h`` 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 |
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a ``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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================================================== |
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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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================================== |
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:: |
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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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======== |
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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 |
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local atomic operations: it makes sure that preemption is disabled around write |
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access to 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 use |
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``this_cpu_ptr()`` instead:: |
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local_inc(this_cpu_ptr(&counters)); |
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Reading the counters |
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==================== |
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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 |
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``local.h``:: |
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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(this_cpu_ptr(&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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mod_timer(&test_timer, jiffies + 1000); |
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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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timer_setup(&test_timer, do_test_timer, 0); |
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mod_timer(&test_timer, jiffies + 1); |
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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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