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* 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
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#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 {
// 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);
// manages a FIFO queue of states
template<typename T> class StateQueue {
virtual ~StateQueue();
// 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; }
// 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; }
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; // written by observer to acknowledge advance of next, read by mutator
// only used by observer
const T* mCurrent; // most recent value returned by poll()
// only used by mutator
T* mMutating; // where updates by mutator are done in place
const T* mExpecting; // what the mutator expects mAck to be set to
bool mInMutation; // whether we're currently in the middle of a mutation
bool mIsDirty; // whether mutating state has been modified since last push
bool mIsInitialized; // whether mutating state has been initialized yet
StateQueueObserverDump mObserverDummyDump; // default area for observer dump if not set
StateQueueObserverDump* mObserverDump; // pointer to active observer dump, always non-NULL
StateQueueMutatorDump mMutatorDummyDump; // default area for mutator dump if not set
StateQueueMutatorDump* mMutatorDump; // pointer to active mutator dump, always non-NULL
}; // class StateQueue
} // namespace android