services/audioflinger/fastpath/StateQueue.h

/*
 * Copyright (C) 2012 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#pragma once

#include <stdatomic.h>

// The state queue template class was originally driven by this use case / requirements:
//  There are two threads: a fast mixer, and a normal mixer, and they share state.
//  The interesting part of the shared state is a set of active fast tracks,
//  and the output HAL configuration (buffer size in frames, sample rate, etc.).
//  Fast mixer thread:
//      periodic with typical period < 10 ms
//      FIFO/RR scheduling policy and a low fixed priority
//      ok to block for bounded time using nanosleep() to achieve desired period
//      must not block on condition wait, mutex lock, atomic operation spin, I/O, etc.
//        under typical operations of mixing, writing, or adding/removing tracks
//      ok to block for unbounded time when the output HAL configuration changes,
//        and this may result in an audible artifact
//      needs read-only access to a recent stable state,
//        but not necessarily the most current one
//      only allocate and free memory when configuration changes
//      avoid conventional logging, as this is a form of I/O and could block
//      defer computation to other threads when feasible; for example
//        cycle times are collected by fast mixer thread but the floating-point
//        statistical calculations on these cycle times are computed by normal mixer
//      these requirements also apply to callouts such as AudioBufferProvider and VolumeProvider
//  Normal mixer thread:
//      periodic with typical period ~20 ms
//      SCHED_OTHER scheduling policy and nice priority == urgent audio
//      ok to block, but prefer to avoid as much as possible
//      needs read/write access to state
//  The normal mixer may need to temporarily suspend the fast mixer thread during mode changes.
//  It will do this using the state -- one of the fields tells the fast mixer to idle.

// Additional requirements:
//  - observer must always be able to poll for and view the latest pushed state; it must never be
//    blocked from seeing that state
//  - observer does not need to see every state in sequence; it is OK for it to skip states
//    [see below for more on this]
//  - mutator must always be able to read/modify a state, it must never be blocked from reading or
//    modifying state
//  - reduce memcpy where possible
//  - work well if the observer runs more frequently than the mutator,
//    as is the case with fast mixer/normal mixer.
// It is not a requirement to work well if the roles were reversed,
// and the mutator were to run more frequently than the observer.
// In this case, the mutator could get blocked waiting for a slot to fill up for
// it to work with. This could be solved somewhat by increasing the depth of the queue, but it would
// still limit the mutator to a finite number of changes before it would block.  A future
// possibility, not implemented here, would be to allow the mutator to safely overwrite an already
// pushed state. This could be done by the mutator overwriting mNext, but then being prepared to
// read an mAck which is actually for the earlier mNext (since there is a race).

// Solution:
//  Let's call the fast mixer thread the "observer" and normal mixer thread the "mutator".
//  We assume there is only a single observer and a single mutator; this is critical.
//  Each state is of type <T>, and should contain only POD (Plain Old Data) and raw pointers, as
//  memcpy() may be used to copy state, and the destructors are run in unpredictable order.
//  The states in chronological order are: previous, current, next, and mutating:
//      previous    read-only, observer can compare vs. current to see the subset that changed
//      current     read-only, this is the primary state for observer
//      next        read-only, when observer is ready to accept a new state it will shift it in:
//                      previous = current
//                      current = next
//                  and the slot formerly used by previous is now available to the mutator.
//      mutating    invisible to observer, read/write to mutator
//  Initialization is tricky, especially for the observer.  If the observer starts execution
//  before the mutator, there are no previous, current, or next states.  And even if the observer
//  starts execution after the mutator, there is a next state but no previous or current states.
//  To solve this, we'll have the observer idle until there is a next state,
//  and it will have to deal with the case where there is no previous state.
//  The states are stored in a shared FIFO queue represented using a circular array.
//  The observer polls for mutations, and receives a new state pointer after a
//  a mutation is pushed onto the queue.  To the observer, the state pointers are
//  effectively in random order, that is the observer should not do address
//  arithmetic on the state pointers.  However to the mutator, the state pointers
//  are in a definite circular order.

#include "Configuration.h"

namespace android {

#ifdef STATE_QUEUE_DUMP
// The StateQueueObserverDump and StateQueueMutatorDump keep
// a cache of StateQueue statistics that can be logged by dumpsys.
// Each individual native word-sized field is accessed atomically.  But the
// overall structure is non-atomic, that is there may be an inconsistency between fields.
// No barriers or locks are used for either writing or reading.
// Only POD types are permitted, and the contents shouldn't be trusted (i.e. do range checks).
// It has a different lifetime than the StateQueue, and so it can't be a member of StateQueue.

struct StateQueueObserverDump {
    StateQueueObserverDump() : mStateChanges(0) { }
    /*virtual*/ ~StateQueueObserverDump() { }
    unsigned    mStateChanges;    // incremented each time poll() detects a state change
    void        dump(int fd);
};

struct StateQueueMutatorDump {
    StateQueueMutatorDump() : mPushDirty(0), mPushAck(0), mBlockedSequence(0) { }
    /*virtual*/ ~StateQueueMutatorDump() { }
    unsigned    mPushDirty;       // incremented each time push() is called with a dirty state
    unsigned    mPushAck;         // incremented each time push(BLOCK_UNTIL_ACKED) is called
    unsigned    mBlockedSequence; // incremented before and after each time that push()
                                  // blocks for more than one PUSH_BLOCK_ACK_NS;
                                  // if odd, then mutator is currently blocked inside push()
    void        dump(int fd);
};
#endif

// manages a FIFO queue of states
// marking as final to avoid derived classes as there are no virtuals.
template<typename T> class StateQueue final {

public:
    // Observer APIs

    // Poll for a state change.  Returns a pointer to a read-only state,
    // or NULL if the state has not been initialized yet.
    // If a new state has not pushed by mutator since the previous poll,
    // then the returned pointer will be unchanged.
    // The previous state pointer is guaranteed to still be valid;
    // this allows the observer to diff the previous and new states.
    const T* poll();

    // Mutator APIs

    // Begin a mutation.  Returns a pointer to a read/write state, except the
    // first time it is called the state is write-only and _must_ be initialized.
    // Mutations cannot be nested.
    // If the state is dirty and has not been pushed onto the state queue yet, then
    // this new mutation will be squashed together with the previous one.
    T*      begin();

    // End the current mutation and indicate whether caller modified the state.
    // If didModify is true, then the state is marked dirty (in need of pushing).
    // There is no rollback option because modifications are done in place.
    // Does not automatically push the new state onto the state queue.
    void    end(bool didModify = true);

    // Push a new state, if any, out to the observer via the state queue.
    // For BLOCK_NEVER, returns:
    //      true if not dirty, or dirty and pushed successfully
    //      false if dirty and not pushed because that would block; remains dirty
    // For BLOCK_UNTIL_PUSHED and BLOCK_UNTIL_ACKED, always returns true.
    // No-op if there are no pending modifications (not dirty), except
    //      for BLOCK_UNTIL_ACKED it will wait until a prior push has been acknowledged.
    // Must not be called in the middle of a mutation.
    enum block_t {
        BLOCK_NEVER,        // do not block
        BLOCK_UNTIL_PUSHED, // block until there's a slot available for the push
        BLOCK_UNTIL_ACKED,  // also block until the push is acknowledged by the observer
    };
    bool    push(block_t block = BLOCK_NEVER);

    // Return whether the current state is dirty (modified and not pushed).
    bool    isDirty() const { return mIsDirty; }

#ifdef STATE_QUEUE_DUMP
    // Register location of observer dump area
    void    setObserverDump(StateQueueObserverDump *dump)
            { mObserverDump = dump != NULL ? dump : &mObserverDummyDump; }

    // Register location of mutator dump area
    void    setMutatorDump(StateQueueMutatorDump *dump)
            { mMutatorDump = dump != NULL ? dump : &mMutatorDummyDump; }
#endif

private:
    static const unsigned kN = 4;       // values < 4 are not supported by this code
    T                 mStates[kN];      // written by mutator, read by observer

    // "volatile" is meaningless with SMP, but here it indicates that we're using atomic ops
    atomic_uintptr_t  mNext{}; // written by mutator to advance next, read by observer
    volatile const T* mAck = nullptr;  // written by observer to acknowledge advance of next,
                                       // read by mutator

    // only used by observer
    const T*    mCurrent = nullptr;     // most recent value returned by poll()

    // only used by mutator
    T*          mMutating{&mStates[0]}; // where updates by mutator are done in place
    const T*    mExpecting = nullptr;   // what the mutator expects mAck to be set to
    bool        mInMutation = false;    // whether we're currently in the middle of a mutation
    bool        mIsDirty = false;       // whether mutating state has been modified since last push
    bool        mIsInitialized = false; // whether mutating state has been initialized yet

#ifdef STATE_QUEUE_DUMP
    StateQueueObserverDump  mObserverDummyDump; // default area for observer dump if not set
    // pointer to active observer dump, always non-nullptr
    StateQueueObserverDump* mObserverDump{&mObserverDummyDump};
    StateQueueMutatorDump   mMutatorDummyDump;  // default area for mutator dump if not set
    // pointer to active mutator dump, always non-nullptr
    StateQueueMutatorDump*  mMutatorDump{&mMutatorDummyDump};
#endif

};  // class StateQueue

}   // namespace android