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    SubjectRe: [PATCH v2 3/4] Documentation/local_ops.txt: convert to ReST markup
    Em Fri, 25 Nov 2016 11:02:41 +0100
    Silvio Fricke <silvio.fricke@gmail.com> escreveu:

    > ... and move to core-api folder.
    >
    > Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com>
    > ---
    > Documentation/core-api/index.rst | 1 +-
    > Documentation/local_ops.txt => Documentation/core-api/local_ops.rst | 275 +++----
    > 2 files changed, 147 insertions(+), 129 deletions(-)
    >
    > 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/local_ops.txt b/Documentation/core-api/local_ops.rst
    > similarity index 55%
    > rename from Documentation/local_ops.txt
    > rename to Documentation/core-api/local_ops.rst
    > index 407576a..01f1880 100644
    > --- a/Documentation/local_ops.txt
    > +++ b/Documentation/core-api/local_ops.rst
    > @@ -1,191 +1,208 @@
    > - Semantics and Behavior of Local Atomic Operations
    >
    > - Mathieu Desnoyers
    > +.. _local_ops:
    >
    > +=================================================
    > +Semantics and Behavior of Local Atomic Operations
    > +=================================================
    >
    > - This document explains the purpose of the local atomic operations, how
    > +: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 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 -Most uses of local_t in the kernel have been replaced by this_cpu operations.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.
    > +.. 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
    > +
    > +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
    > +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.
    > +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
    > +Implementation for a given architecture
    > +=======================================
    >
    > -It can be done by slightly modifying the standard atomic operations : only
    > +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.
    > +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 :
    > +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;
    > + typedef struct { atomic_long_t a; } local_t;
    >
    >
    > -* Rules to follow when using local atomic operations
    > +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
    > +* 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
    > +* 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
    > +* 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
    > +* 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.
    > + variable can be read when reading some *other* cpu's variables.
    > +
    >
    > +How to use local atomic operations
    > +==================================
    >
    > -* How to use local atomic operations
    > +.. code-block:: c

    Better to use :: instead of code-block.

    >
    > -#include <linux/percpu.h>
    > -#include <asm/local.h>
    > + #include <linux/percpu.h>
    > + #include <asm/local.h>
    >
    > -static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
    > + static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
    >
    >
    > -* Counting
    > +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 :
    > +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);
    > + 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.
    > +``this_cpu_ptr()`` instead. ::
    >
    > - local_inc(this_cpu_ptr(&counters));
    > + local_inc(this_cpu_ptr(&counters));
    >
    >
    >
    > -* Reading the 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.
    > +relatively to other memory writes happening on the CPU that owns the data. ::

    Please remove the dot at the end.

    >
    > - long sum = 0;
    > - for_each_online_cpu(cpu)
    > - sum += local_read(&per_cpu(counters, cpu));
    > + 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
    > +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 ---
    > +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

    Better to use :: instead of code-block.

    > +
    > + /* 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");



    Thanks,
    Mauro

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