StateQueue.h revision 639482c24c911b125398b31883ba6d55faebe28b
1/* 2 * Copyright (C) 2012 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17#ifndef ANDROID_AUDIO_STATE_QUEUE_H 18#define ANDROID_AUDIO_STATE_QUEUE_H 19 20// The state queue template class was originally driven by this use case / requirements: 21// There are two threads: a fast mixer, and a normal mixer, and they share state. 22// The interesting part of the shared state is a set of active fast tracks, 23// and the output HAL configuration (buffer size in frames, sample rate, etc.). 24// Fast mixer thread: 25// periodic with typical period < 10 ms 26// FIFO/RR scheduling policy and a low fixed priority 27// ok to block for bounded time using nanosleep() to achieve desired period 28// must not block on condition wait, mutex lock, atomic operation spin, I/O, etc. 29// under typical operations of mixing, writing, or adding/removing tracks 30// ok to block for unbounded time when the output HAL configuration changes, 31// and this may result in an audible artifact 32// needs read-only access to a recent stable state, 33// but not necessarily the most current one 34// Normal mixer thread: 35// periodic with typical period ~40 ms 36// SCHED_OTHER scheduling policy and nice priority == urgent audio 37// ok to block, but prefer to avoid as much as possible 38// needs read/write access to state 39// The normal mixer may need to temporarily suspend the fast mixer thread during mode changes. 40// It will do this using the state -- one of the fields tells the fast mixer to idle. 41 42// Additional requirements: 43// - observer must always be able to poll for and view the latest pushed state; it must never be 44// blocked from seeing that state 45// - observer does not need to see every state in sequence; it is OK for it to skip states 46// [see below for more on this] 47// - mutator must always be able to read/modify a state, it must never be blocked from reading or 48// modifying state 49// - reduce memcpy where possible 50// - work well if the observer runs more frequently than the mutator, 51// as is the case with fast mixer/normal mixer. 52// It is not a requirement to work well if the roles were reversed, 53// and the mutator were to run more frequently than the observer. 54// In this case, the mutator could get blocked waiting for a slot to fill up for 55// it to work with. This could be solved somewhat by increasing the depth of the queue, but it would 56// still limit the mutator to a finite number of changes before it would block. A future 57// possibility, not implemented here, would be to allow the mutator to safely overwrite an already 58// pushed state. This could be done by the mutator overwriting mNext, but then being prepared to 59// read an mAck which is actually for the earlier mNext (since there is a race). 60 61// Solution: 62// Let's call the fast mixer thread the "observer" and normal mixer thread the "mutator". 63// We assume there is only a single observer and a single mutator; this is critical. 64// Each state is of type <T>, and should contain only POD (Plain Old Data) and raw pointers, as 65// memcpy() may be used to copy state, and the destructors are run in unpredictable order. 66// The states in chronological order are: previous, current, next, and mutating: 67// previous read-only, observer can compare vs. current to see the subset that changed 68// current read-only, this is the primary state for observer 69// next read-only, when observer is ready to accept a new state it will shift it in: 70// previous = current 71// current = next 72// and the slot formerly used by previous is now available to the mutator. 73// mutating invisible to observer, read/write to mutator 74// Initialization is tricky, especially for the observer. If the observer starts execution 75// before the mutator, there are no previous, current, or next states. And even if the observer 76// starts execution after the mutator, there is a next state but no previous or current states. 77// To solve this, we'll have the observer idle until there is a next state, 78// and it will have to deal with the case where there is no previous state. 79// The states are stored in a shared FIFO queue represented using a circular array. 80// The observer polls for mutations, and receives a new state pointer after a 81// a mutation is pushed onto the queue. To the observer, the state pointers are 82// effectively in random order, that is the observer should not do address 83// arithmetic on the state pointers. However to the mutator, the state pointers 84// are in a definite circular order. 85 86namespace android { 87 88#ifdef STATE_QUEUE_DUMP 89// The StateQueueObserverDump and StateQueueMutatorDump keep 90// a cache of StateQueue statistics that can be logged by dumpsys. 91// Each individual native word-sized field is accessed atomically. But the 92// overall structure is non-atomic, that is there may be an inconsistency between fields. 93// No barriers or locks are used for either writing or reading. 94// Only POD types are permitted, and the contents shouldn't be trusted (i.e. do range checks). 95// It has a different lifetime than the StateQueue, and so it can't be a member of StateQueue. 96 97struct StateQueueObserverDump { 98 StateQueueObserverDump() : mStateChanges(0) { } 99 /*virtual*/ ~StateQueueObserverDump() { } 100 unsigned mStateChanges; // incremented each time poll() detects a state change 101 void dump(int fd); 102}; 103 104struct StateQueueMutatorDump { 105 StateQueueMutatorDump() : mPushDirty(0), mPushAck(0), mBlockedSequence(0) { } 106 /*virtual*/ ~StateQueueMutatorDump() { } 107 unsigned mPushDirty; // incremented each time push() is called with a dirty state 108 unsigned mPushAck; // incremented each time push(BLOCK_UNTIL_ACKED) is called 109 unsigned mBlockedSequence; // incremented before and after each time that push() 110 // blocks for more than one PUSH_BLOCK_ACK_NS; 111 // if odd, then mutator is currently blocked inside push() 112 void dump(int fd); 113}; 114#endif 115 116// manages a FIFO queue of states 117template<typename T> class StateQueue { 118 119public: 120 StateQueue(); 121 virtual ~StateQueue(); 122 123 // Observer APIs 124 125 // Poll for a state change. Returns a pointer to a read-only state, 126 // or NULL if the state has not been initialized yet. 127 // If a new state has not pushed by mutator since the previous poll, 128 // then the returned pointer will be unchanged. 129 // The previous state pointer is guaranteed to still be valid; 130 // this allows the observer to diff the previous and new states. 131 const T* poll(); 132 133 // Mutator APIs 134 135 // Begin a mutation. Returns a pointer to a read/write state, except the 136 // first time it is called the state is write-only and _must_ be initialized. 137 // Mutations cannot be nested. 138 // If the state is dirty and has not been pushed onto the state queue yet, then 139 // this new mutation will be squashed together with the previous one. 140 T* begin(); 141 142 // End the current mutation and indicate whether caller modified the state. 143 // If didModify is true, then the state is marked dirty (in need of pushing). 144 // There is no rollback option because modifications are done in place. 145 // Does not automatically push the new state onto the state queue. 146 void end(bool didModify = true); 147 148 // Push a new state, if any, out to the observer via the state queue. 149 // For BLOCK_NEVER, returns: 150 // true if not dirty, or dirty and pushed successfully 151 // false if dirty and not pushed because that would block; remains dirty 152 // For BLOCK_UNTIL_PUSHED and BLOCK_UNTIL_ACKED, always returns true. 153 // No-op if there are no pending modifications (not dirty), except 154 // for BLOCK_UNTIL_ACKED it will wait until a prior push has been acknowledged. 155 // Must not be called in the middle of a mutation. 156 enum block_t { 157 BLOCK_NEVER, // do not block 158 BLOCK_UNTIL_PUSHED, // block until there's a slot available for the push 159 BLOCK_UNTIL_ACKED, // also block until the push is acknowledged by the observer 160 }; 161 bool push(block_t block = BLOCK_NEVER); 162 163 // Return whether the current state is dirty (modified and not pushed). 164 bool isDirty() const { return mIsDirty; } 165 166#ifdef STATE_QUEUE_DUMP 167 // Register location of observer dump area 168 void setObserverDump(StateQueueObserverDump *dump) 169 { mObserverDump = dump != NULL ? dump : &mObserverDummyDump; } 170 171 // Register location of mutator dump area 172 void setMutatorDump(StateQueueMutatorDump *dump) 173 { mMutatorDump = dump != NULL ? dump : &mMutatorDummyDump; } 174#endif 175 176private: 177 static const unsigned kN = 8; // values < 4 are not supported by this code 178 T mStates[kN]; // written by mutator, read by observer 179 180 // "volatile" is meaningless with SMP, but here it indicates that we're using atomic ops 181 volatile const T* mNext; // written by mutator to advance next, read by observer 182 volatile const T* mAck; // written by observer to acknowledge advance of next, read by mutator 183 184 // only used by observer 185 const T* mCurrent; // most recent value returned by poll() 186 187 // only used by mutator 188 T* mMutating; // where updates by mutator are done in place 189 const T* mExpecting; // what the mutator expects mAck to be set to 190 bool mInMutation; // whether we're currently in the middle of a mutation 191 bool mIsDirty; // whether mutating state has been modified since last push 192 bool mIsInitialized; // whether mutating state has been initialized yet 193 194#ifdef STATE_QUEUE_DUMP 195 StateQueueObserverDump mObserverDummyDump; // default area for observer dump if not set 196 StateQueueObserverDump* mObserverDump; // pointer to active observer dump, always non-NULL 197 StateQueueMutatorDump mMutatorDummyDump; // default area for mutator dump if not set 198 StateQueueMutatorDump* mMutatorDump; // pointer to active mutator dump, always non-NULL 199#endif 200 201}; // class StateQueue 202 203} // namespace android 204 205#endif // ANDROID_AUDIO_STATE_QUEUE_H 206