Documentation/local_ops.txt: convert to ReST markup (c232694e) · Commits · e / devices / android_kernel_teracube_emerald

Documentation/core-api/index.rst

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		@@ -8,6 +8,7 @@ Kernel and driver related documentation.
		:maxdepth: 1

		assoc_array
		local_ops
		workqueue

		.. only:: subproject

Documentation/core-api/local_ops.rst

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		.. _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 Most uses of ``local_t`` in the kernel have been
		replaced by ``this_cpu`` operations.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");

Documentation/local_ops.txt

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Original line number	Diff line number	Diff line
		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.

		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.

		--- 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(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");
		--- END ---