Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754490AbcKYO2h (ORCPT ); Fri, 25 Nov 2016 09:28:37 -0500 Received: from ec2-52-27-115-49.us-west-2.compute.amazonaws.com ([52.27.115.49]:59659 "EHLO osg.samsung.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753411AbcKYO23 (ORCPT ); Fri, 25 Nov 2016 09:28:29 -0500 Date: Fri, 25 Nov 2016 11:34:06 -0200 From: Mauro Carvalho Chehab To: Silvio Fricke Cc: linux-doc@vger.kernel.org, Jonathan Corbet , Ming Lei , "Luis R . Rodriguez" , Mauro Carvalho Chehab , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 3/4] Documentation/local_ops.txt: convert to ReST markup Message-ID: <20161125113406.3be24b9d@vento.lan> In-Reply-To: <3091d1d552cb9673cea790906469a511becb63a1.1480068123.git-series.silvio.fricke@gmail.com> References: <3091d1d552cb9673cea790906469a511becb63a1.1480068123.git-series.silvio.fricke@gmail.com> Organization: Samsung X-Mailer: Claws Mail 3.14.0 (GTK+ 2.24.31; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 14536 Lines: 382 Em Fri, 25 Nov 2016 11:02:41 +0100 Silvio Fricke escreveu: > ... and move to core-api folder. > > Signed-off-by: Silvio Fricke > --- > 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 > -#include > + #include > + #include > > -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 > -#include > -#include > - > -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 > + #include > + #include > + > + 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