Message-ID: <20161125134711.GC14150@sfserver>
Date:   Fri, 25 Nov 2016 14:47:11 +0100
From:   "S. Fricke" <silvio.fricke@...il.com>
To:     Mauro Carvalho Chehab <mchehab@...pensource.com>
Cc:     linux-doc@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Ming Lei <ming.lei@...onical.com>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/4] Documentation/local_ops.txt: convert to ReST
 markup

Hi Mauro,

I have a question about the "code-block" and "::". On which situation should
I use "code-block" and on which condition a "::"?
For now I have used "::" on small one, two or three liners, and "code-block"
for "example code" snippets or longer code segments.

Thanks for a small clarification,
Silvio

> Em Fri, 25 Nov 2016 11:02:41 +0100
> Silvio Fricke <silvio.fricke@...il.com> escreveu:
> 
> > ... and move to core-api folder.
> > 
> > Signed-off-by: Silvio Fricke <silvio.fricke@...il.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

-- 
-- S. Fricke ---------------------------------------- silvio@...t1024.net --
   Diplom-Informatiker (FH)
   Linux-Development                         Matrix: @silvio:port1024.net   
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