From: Honnappa Nagarahalli Date: Wed, 1 May 2019 03:54:18 +0000 (-0500) Subject: doc: add RCU guide X-Git-Url: http://git.droids-corp.org/?p=dpdk.git;a=commitdiff_plain;h=d67e6ecf46e25832c4b24dba76c0a9dcd3e0dc84 doc: add RCU guide Add lib RCU QSBR programmer guide documentation. Signed-off-by: Honnappa Nagarahalli Reviewed-by: Marko Kovacevic Acked-by: John McNamara --- diff --git a/doc/guides/prog_guide/img/rcu_general_info.svg b/doc/guides/prog_guide/img/rcu_general_info.svg new file mode 100644 index 0000000000..e7ca1dacb9 --- /dev/null +++ b/doc/guides/prog_guide/img/rcu_general_info.svg @@ -0,0 +1,509 @@ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Page-1 + + + + Sheet.3 + + + + Sheet.4 + + + + Sheet.5 + D1 + + + + D1 + + Sheet.6 + + + + Sheet.7 + + + + Sheet.8 + D2 + + + + D2 + + Sheet.9 + + + + Sheet.10 + Reader Thread 1 + + + + Reader Thread 1 + + Sheet.11 + + + + Sheet.12 + + + + Sheet.13 + D1 + + + + D1 + + Sheet.14 + + + + Sheet.15 + + + + Sheet.16 + D2 + + + + D2 + + Sheet.17 + + + + Sheet.18 + T 2 + + + + T 2 + + Sheet.19 + + + + Sheet.20 + + + + Sheet.21 + D1 + + + + D1 + + Sheet.22 + + + + Sheet.23 + + + + Sheet.24 + D2 + + + + D2 + + Sheet.25 + + + + Sheet.26 + T 3 + + + + T 3 + + Sheet.28 + Time + + + + Time + + Sheet.29 + + + + Sheet.30 + + + + Sheet.31 + Remove reference to entry1 + + + + Remove reference to entry1 + + Sheet.33 + + + + Sheet.34 + Delete + + + + Delete + + Sheet.35 + + + + Sheet.36 + + + + Sheet.37 + Delete entry1 from D1 + + + + Delete entry1 from D1 + + Sheet.38 + + + + Sheet.39 + + + + Sheet.40 + + + + Sheet.41 + Free memory for entries1 and 2 after every reader has gone th... + + + + Free memory for entries1 and 2 after every reader has gone through at least 1 quiescent state + + Sheet.46 + Free + + + + Free + + Sheet.48 + + + + Sheet.49 + + + + Sheet.50 + Grace Period + + + + Grace Period + + Sheet.51 + + + + Sheet.52 + + + + Sheet.53 + + + + Sheet.54 + Delete entry2 from D1 + + + + Delete entry2 from D1 + + Sheet.56 + + + + Sheet.57 + + + + Sheet.58 + + + + Sheet.59 + Critical sections + + + + Critical sections + + Sheet.60 + + + + Sheet.61 + + + + Sheet.62 + + + + Sheet.63 + Quiescent states + + + + Quiescent states + + Sheet.64 + + + + Sheet.65 + + + + Sheet.66 + + + + Sheet.67 + + + + Sheet.68 + + + + Sheet.69 + + + + Sheet.70 + while(1) loop + + + + while(1) loop + + Sheet.71 + + + + Sheet.72 + Reader thread is not accessing any shared data structure. i.e... + + + + Reader thread is not accessing any shared data structure.i.e. non critical section or quiescent state. + + Sheet.73 + Dx + + + + Dx + + Sheet.74 + Reader thread is accessing the shared data structure Dx. i.e.... + + + + Reader thread is accessing the shared data structure Dx.i.e. critical section. + + Sheet.75 + Point in time when the reference to the entry is removed usin... + + + + Point in time when the reference to the entry is removed using an atomic operation. + + Sheet.76 + Delete + + + + Delete + + Sheet.77 + Point in time when the writer can free the deleted entry. + + + + Point in time when the writer can free the deleted entry. + + Sheet.78 + Free + + + + Free + + Sheet.79 + Time duration between Delete and Free, during which memory ca... + + + + Time duration between Delete and Free, during which memory cannot be freed. + + Sheet.80 + Grace Period + + + + Grace Period + + Sheet.83 + + + + Sheet.84 + + + + Sheet.85 + + + + Sheet.86 + + + + Sheet.87 + + + + Sheet.88 + Remove reference to entry2 + + + + Remove reference to entry2 + + diff --git a/doc/guides/prog_guide/index.rst b/doc/guides/prog_guide/index.rst index 95f5e7964d..692409af82 100644 --- a/doc/guides/prog_guide/index.rst +++ b/doc/guides/prog_guide/index.rst @@ -12,6 +12,7 @@ Programmer's Guide overview env_abstraction_layer service_cores + rcu_lib ring_lib stack_lib mempool_lib diff --git a/doc/guides/prog_guide/rcu_lib.rst b/doc/guides/prog_guide/rcu_lib.rst new file mode 100644 index 0000000000..8fe5b1f738 --- /dev/null +++ b/doc/guides/prog_guide/rcu_lib.rst @@ -0,0 +1,188 @@ +.. SPDX-License-Identifier: BSD-3-Clause + Copyright(c) 2019 Arm Limited. + +.. _RCU_Library: + +RCU Library +============ + +Lockless data structures provide scalability and determinism. +They enable use cases where locking may not be allowed +(for example real-time applications). + +In the following sections, the term "memory" refers to memory allocated +by typical APIs like malloc() or anything that is representative of +memory, for example an index of a free element array. + +Since these data structures are lockless, the writers and readers +are accessing the data structures concurrently. Hence, while removing +an element from a data structure, the writers cannot return the memory +to the allocator, without knowing that the readers are not +referencing that element/memory anymore. Hence, it is required to +separate the operation of removing an element into two steps: + +#. Delete: in this step, the writer removes the reference to the element from + the data structure but does not return the associated memory to the + allocator. This will ensure that new readers will not get a reference to + the removed element. Removing the reference is an atomic operation. + +#. Free (Reclaim): in this step, the writer returns the memory to the + memory allocator only after knowing that all the readers have stopped + referencing the deleted element. + +This library helps the writer determine when it is safe to free the +memory by making use of thread Quiescent State (QS). + +What is Quiescent State +----------------------- + +Quiescent State can be defined as "any point in the thread execution where the +thread does not hold a reference to shared memory". It is up to the application +to determine its quiescent state. + +Let us consider the following diagram: + +.. _figure_quiescent_state: + +.. figure:: img/rcu_general_info.* + + Phases in the Quiescent State model. + + +As shown in :numref:`figure_quiescent_state`, reader thread 1 accesses data +structures D1 and D2. When it is accessing D1, if the writer has to remove an +element from D1, the writer cannot free the memory associated with that +element immediately. The writer can return the memory to the allocator only +after the reader stops referencing D1. In other words, reader thread RT1 has +to enter a quiescent state. + +Similarly, since reader thread 2 is also accessing D1, the writer has to +wait till thread 2 enters quiescent state as well. + +However, the writer does not need to wait for reader thread 3 to enter +quiescent state. Reader thread 3 was not accessing D1 when the delete +operation happened. So, reader thread 1 will not have a reference to the +deleted entry. + +It can be noted that, the critical sections for D2 is a quiescent state +for D1. i.e. for a given data structure Dx, any point in the thread execution +that does not reference Dx is a quiescent state. + +Since memory is not freed immediately, there might be a need for +provisioning of additional memory, depending on the application requirements. + +Factors affecting the RCU mechanism +----------------------------------- + +It is important to make sure that this library keeps the overhead of +identifying the end of grace period and subsequent freeing of memory, +to a minimum. The following explains how grace period and critical +section affect this overhead. + +The writer has to poll the readers to identify the end of grace period. +Polling introduces memory accesses and wastes CPU cycles. The memory +is not available for reuse during the grace period. Longer grace periods +exasperate these conditions. + +The length of the critical section and the number of reader threads +is proportional to the duration of the grace period. Keeping the critical +sections smaller will keep the grace period smaller. However, keeping the +critical sections smaller requires additional CPU cycles (due to additional +reporting) in the readers. + +Hence, we need the characteristics of a small grace period and large critical +section. This library addresses this by allowing the writer to do +other work without having to block until the readers report their quiescent +state. + +RCU in DPDK +----------- + +For DPDK applications, the start and end of a ``while(1)`` loop (where no +references to shared data structures are kept) act as perfect quiescent +states. This will combine all the shared data structure accesses into a +single, large critical section which helps keep the overhead on the +reader side to a minimum. + +DPDK supports a pipeline model of packet processing and service cores. +In these use cases, a given data structure may not be used by all the +workers in the application. The writer does not have to wait for all +the workers to report their quiescent state. To provide the required +flexibility, this library has a concept of a QS variable. The application +can create one QS variable per data structure to help it track the +end of grace period for each data structure. This helps keep the grace +period to a minimum. + +How to use this library +----------------------- + +The application must allocate memory and initialize a QS variable. + +Applications can call ``rte_rcu_qsbr_get_memsize()`` to calculate the size +of memory to allocate. This API takes a maximum number of reader threads, +using this variable, as a parameter. Currently, a maximum of 1024 threads +are supported. + +Further, the application can initialize a QS variable using the API +``rte_rcu_qsbr_init()``. + +Each reader thread is assumed to have a unique thread ID. Currently, the +management of the thread ID (for example allocation/free) is left to the +application. The thread ID should be in the range of 0 to +maximum number of threads provided while creating the QS variable. +The application could also use ``lcore_id`` as the thread ID where applicable. + +The ``rte_rcu_qsbr_thread_register()`` API will register a reader thread +to report its quiescent state. This can be called from a reader thread. +A control plane thread can also call this on behalf of a reader thread. +The reader thread must call ``rte_rcu_qsbr_thread_online()`` API to start +reporting its quiescent state. + +Some of the use cases might require the reader threads to make blocking API +calls (for example while using eventdev APIs). The writer thread should not +wait for such reader threads to enter quiescent state. The reader thread must +call ``rte_rcu_qsbr_thread_offline()`` API, before calling blocking APIs. It +can call ``rte_rcu_qsbr_thread_online()`` API once the blocking API call +returns. + +The writer thread can trigger the reader threads to report their quiescent +state by calling the API ``rte_rcu_qsbr_start()``. It is possible for multiple +writer threads to query the quiescent state status simultaneously. Hence, +``rte_rcu_qsbr_start()`` returns a token to each caller. + +The writer thread must call ``rte_rcu_qsbr_check()`` API with the token to +get the current quiescent state status. Option to block till all the reader +threads enter the quiescent state is provided. If this API indicates that +all the reader threads have entered the quiescent state, the application +can free the deleted entry. + +The APIs ``rte_rcu_qsbr_start()`` and ``rte_rcu_qsbr_check()`` are lock free. +Hence, they can be called concurrently from multiple writers even while +running as worker threads. + +The separation of triggering the reporting from querying the status provides +the writer threads flexibility to do useful work instead of blocking for the +reader threads to enter the quiescent state or go offline. This reduces the +memory accesses due to continuous polling for the status. + +The ``rte_rcu_qsbr_synchronize()`` API combines the functionality of +``rte_rcu_qsbr_start()`` and blocking ``rte_rcu_qsbr_check()`` into a single +API. This API triggers the reader threads to report their quiescent state and +polls till all the readers enter the quiescent state or go offline. This API +does not allow the writer to do useful work while waiting and introduces +additional memory accesses due to continuous polling. + +The reader thread must call ``rte_rcu_qsbr_thread_offline()`` and +``rte_rcu_qsbr_thread_unregister()`` APIs to remove itself from reporting its +quiescent state. The ``rte_rcu_qsbr_check()`` API will not wait for this reader +thread to report the quiescent state status anymore. + +The reader threads should call ``rte_rcu_qsbr_quiescent()`` API to indicate that +they entered a quiescent state. This API checks if a writer has triggered a +quiescent state query and update the state accordingly. + +The ``rte_rcu_qsbr_lock()`` and ``rte_rcu_qsbr_unlock()`` are empty functions. +However, when ``CONFIG_RTE_LIBRTE_RCU_DEBUG`` is enabled, these APIs aid +in debugging issues. One can mark the access to shared data structures on the +reader side using these APIs. The ``rte_rcu_qsbr_quiescent()`` will check if +all the locks are unlocked.