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+ <g id="shape40-105" v:mID="40" v:groupContext="shape" transform="translate(716.782,-675.425)">
+ <title>Sheet.40</title>
+ <path d="M79.2 1188 L52.71 1187.94 L0 1147.21" class="st7"/>
+ </g>
+ <g id="shape41-108" v:mID="41" v:groupContext="shape" transform="translate(803.572,-639.285)">
+ <title>Sheet.41</title>
+ <desc>Free memory for entries1 and 2 after every reader has gone th...</desc>
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+ <text x="0" y="1133.61" class="st8" v:langID="1033"><v:paragraph/><v:tabList/>Free memory for entries1 and 2 <tspan
+ x="0" dy="1.2em" class="st9">after every reader has gone </tspan><tspan x="0" dy="1.2em" class="st9">through at least 1 quiescent state </tspan> </text> </g>
+ <g id="shape46-114" v:mID="46" v:groupContext="shape" transform="translate(680.801,-1130.65)">
+ <title>Sheet.46</title>
+ <desc>Free</desc>
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+ <title>Sheet.48</title>
+ <path d="M0 1145.17 L0 1188 L0 1145.17" class="st7"/>
+ </g>
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+ <title>Sheet.49</title>
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+ </g>
+ <g id="shape50-124" v:mID="50" v:groupContext="shape" transform="translate(798.359,-1110.46)">
+ <title>Sheet.50</title>
+ <desc>Grace Period</desc>
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+ <text x="43.79" y="1174.99" class="st6" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Grace Period</text> </g>
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+ <title>Sheet.51</title>
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+ <title>Sheet.53</title>
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+ </g>
+ <g id="shape54-137" v:mID="54" v:groupContext="shape" transform="translate(225,-1058.76)">
+ <title>Sheet.54</title>
+ <desc>Delete entry2 from D1</desc>
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+ <text x="3.88" y="1182.06" class="st8" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Delete entry2 from D1</text> </g>
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+ <title>Sheet.56</title>
+ <path d="M0 732.77 L0 1188" class="st12"/>
+ </g>
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+ <path d="M-0 1188 L146.76 1112.94" class="st13"/>
+ </g>
+ <g id="shape58-147" v:mID="58" v:groupContext="shape" transform="translate(619.059,-848.701)">
+ <title>Sheet.58</title>
+ <path d="M432.2 1184.24 L-0 1188" class="st7"/>
+ </g>
+ <g id="shape59-150" v:mID="59" v:groupContext="shape" transform="translate(1038.62,-837.364)">
+ <title>Sheet.59</title>
+ <desc>Critical sections</desc>
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+ <text x="52.25" y="1174.99" class="st6" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Critical sections</text> </g>
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+ <title>Sheet.60</title>
+ <path d="M0 1173.53 L0 1188" class="st7"/>
+ </g>
+ <g id="shape61-157" v:mID="61" v:groupContext="shape" transform="translate(824.31,-849.848)">
+ <title>Sheet.61</title>
+ <path d="M0 1173.53 L0 1188" class="st7"/>
+ </g>
+ <g id="shape62-160" v:mID="62" v:groupContext="shape" transform="translate(345.944,-933.143)">
+ <title>Sheet.62</title>
+ <path d="M705.32 1188 L0 1187.43" class="st7"/>
+ </g>
+ <g id="shape63-163" v:mID="63" v:groupContext="shape" transform="translate(1038.62,-915.684)">
+ <title>Sheet.63</title>
+ <desc>Quiescent states</desc>
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+ <text x="55.18" y="1174.99" class="st6" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Quiescent states</text> </g>
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+ <title>Sheet.64</title>
+ <path d="M0 1173.53 L0 1188" class="st7"/>
+ </g>
+ <g id="shape65-170" v:mID="65" v:groupContext="shape" transform="translate(621.606,-933.461)">
+ <title>Sheet.65</title>
+ <path d="M0 1173.53 L0 1188" class="st7"/>
+ </g>
+ <g id="shape66-173" v:mID="66" v:groupContext="shape" transform="translate(856.905,-934.481)">
+ <title>Sheet.66</title>
+ <path d="M0 1173.53 L0 1188" class="st7"/>
+ </g>
+ <g id="shape67-176" v:mID="67" v:groupContext="shape" transform="translate(472.82,-756.389)">
+ <title>Sheet.67</title>
+ <path d="M578.44 1188 L0 1187.43" class="st7"/>
+ </g>
+ <g id="shape68-179" v:mID="68" v:groupContext="shape" transform="translate(473.456,-755.688)">
+ <title>Sheet.68</title>
+ <path d="M0 1173.53 L0 1188" class="st7"/>
+ </g>
+ <g id="shape69-182" v:mID="69" v:groupContext="shape" transform="translate(1016.87,-757.728)">
+ <title>Sheet.69</title>
+ <path d="M0 1173.53 L0 1188" class="st7"/>
+ </g>
+ <g id="shape70-185" v:mID="70" v:groupContext="shape" transform="translate(1060.04,-738.651)">
+ <title>Sheet.70</title>
+ <desc>while(1) loop</desc>
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+ <text x="31.03" y="1174.99" class="st6" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>while(1) loop</text> </g>
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+ <title>Sheet.71</title>
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+ <g id="shape72-191" v:mID="72" v:groupContext="shape" transform="translate(259.003,-466.895)">
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+ <title>Sheet.73</title>
+ <desc>Dx</desc>
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+ <title>Sheet.74</title>
+ <desc>Reader thread is accessing the shared data structure Dx. i.e....</desc>
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+ <text x="0" y="1162.25" class="st14" v:langID="1033"><v:paragraph/><v:tabList/>Reader thread is accessing the shared data structure Dx.<v:newlineChar/><tspan
+ x="0" dy="1.2em" class="st9">i.e. critical section.</tspan></text> </g>
+ <g id="shape75-204" v:mID="75" v:groupContext="shape" transform="translate(289.017,-301.151)">
+ <title>Sheet.75</title>
+ <desc>Point in time when the reference to the entry is removed usin...</desc>
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+ <text x="0" y="1153.27" class="st14" v:langID="1033"><v:paragraph/><v:tabList/>Point in time when the reference to the entry is removed <tspan
+ x="0" dy="1.2em" class="st9">using an atomic operation.</tspan></text> </g>
+ <g id="shape76-209" v:mID="76" v:groupContext="shape" transform="translate(177.543,-315.596)">
+ <title>Sheet.76</title>
+ <desc>Delete</desc>
+ <v:textBlock v:margins="rect(0,0,0,0)"/>
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+ <text x="11.25" y="1174.2" class="st11" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Delete</text> </g>
+ <g id="shape77-213" v:mID="77" v:groupContext="shape" transform="translate(288,-239.327)">
+ <title>Sheet.77</title>
+ <desc>Point in time when the writer can free the deleted entry.</desc>
+ <v:textBlock v:margins="rect(0,0,0,0)"/>
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+ <text x="0" y="1175.01" class="st14" v:langID="1033"><v:paragraph/><v:tabList/>Point in time when the writer can free the deleted entry.</text> </g>
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+ <desc>Free</desc>
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+ <title>Sheet.79</title>
+ <desc>Time duration between Delete and Free, during which memory ca...</desc>
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+ <text x="0" y="1160.61" class="st14" v:langID="1033"><v:paragraph/><v:tabList/>Time duration between Delete and Free, during which <tspan
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+ <desc>Grace Period</desc>
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+ class="st9">Period</tspan></text> </g>
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+ </g>
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+ </g>
+ <g id="shape88-260" v:mID="88" v:groupContext="shape" transform="translate(241.369,-607.028)">
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+ <desc>Remove reference to entry2</desc>
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+ <text x="2.39" y="1163.15" class="st8" v:langID="1033"><v:paragraph v:horizAlign="2"/><v:tabList/>Remove reference <tspan
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+ </g>
+</svg>
--- /dev/null
+.. 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.
git.droids-corp.org / dpdk.git / commitdiff
