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Diffstat (limited to 'Documentation/core-api')
| -rw-r--r-- | Documentation/core-api/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/core-api/local_ops.rst | 206 |
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diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst index 480d9a323b60..f53555ee4931 100644 --- a/Documentation/core-api/index.rst +++ b/Documentation/core-api/index.rst @@ -8,6 +8,7 @@ Kernel and driver related documentation. :maxdepth: 1 assoc_array + local_ops workqueue .. only:: subproject diff --git a/Documentation/core-api/local_ops.rst b/Documentation/core-api/local_ops.rst new file mode 100644 index 000000000000..1062ddba62c7 --- /dev/null +++ b/Documentation/core-api/local_ops.rst @@ -0,0 +1,206 @@ + +.. _local_ops: + +================================================= +Semantics and Behavior of Local Atomic Operations +================================================= + +:Author: 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. + +.. note:: + + Note that ``local_t`` based operations are not recommended for general + kernel use. Please use the ``this_cpu`` operations instead unless there is + really a special purpose. Most uses of ``local_t`` in the kernel have been + replaced by ``this_cpu`` operations. ``this_cpu`` operations combine the + relocation with the ``local_t`` like semantics in a single instruction and + yield more compact and faster executing code. + + +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 use +``this_cpu_ptr()`` instead:: + + local_inc(this_cpu_ptr(&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``:: + + /* 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(this_cpu_ptr(&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"); |