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