... and move to core-api folder.
Signed-off-by: Silvio Fricke <silvio.fricke@xxxxxxxxx>
---
Documentation/core-api/index.rst | 1 +-
Documentation/core-api/local_ops.rst | 208 ++++++++++++++++++++++++++++-
Documentation/local_ops.txt | 191 +--------------------------
3 files changed, 209 insertions(+), 191 deletions(-)
create mode 100644 Documentation/core-api/local_ops.rst
delete mode 100644 Documentation/local_ops.txt
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index f3e5f5e..25b4e4a 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -9,6 +9,7 @@ Kernel and driver related documentation.
assoc_array
atomic_ops
+ 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 0000000..01f1880
--- /dev/null
+++ b/Documentation/core-api/local_ops.rst
@@ -0,0 +1,208 @@
+
+.. _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
+==================================
+
+.. code-block:: c
+
+ #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``.
+
+.. code-block:: c
+
+ /* 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");
diff --git a/Documentation/local_ops.txt b/Documentation/local_ops.txt
deleted file mode 100644
index 407576a..0000000
--- a/Documentation/local_ops.txt
+++ /dev/null
@@ -1,191 +0,0 @@
- 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 ---
--
git-series 0.9.1
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