1/*
2 * Copyright (C) 2017 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
17package com.android.server;
18
19import static android.os.Process.getThreadPriority;
20import static android.os.Process.myTid;
21import static android.os.Process.setThreadPriority;
22
23/**
24 * Utility class to boost threads in sections where important locks are held.
25 */
26public class ThreadPriorityBooster {
27
28    private static final boolean ENABLE_LOCK_GUARD = false;
29
30    private volatile int mBoostToPriority;
31    private final int mLockGuardIndex;
32
33    private final ThreadLocal<PriorityState> mThreadState = new ThreadLocal<PriorityState>() {
34        @Override protected PriorityState initialValue() {
35            return new PriorityState();
36        }
37    };
38
39    public ThreadPriorityBooster(int boostToPriority, int lockGuardIndex) {
40        mBoostToPriority = boostToPriority;
41        mLockGuardIndex = lockGuardIndex;
42    }
43
44    public void boost() {
45        final int tid = myTid();
46        final int prevPriority = getThreadPriority(tid);
47        final PriorityState state = mThreadState.get();
48        if (state.regionCounter == 0) {
49            state.prevPriority = prevPriority;
50            if (prevPriority > mBoostToPriority) {
51                setThreadPriority(tid, mBoostToPriority);
52            }
53        }
54        state.regionCounter++;
55        if (ENABLE_LOCK_GUARD) {
56            LockGuard.guard(mLockGuardIndex);
57        }
58    }
59
60    public void reset() {
61        final PriorityState state = mThreadState.get();
62        state.regionCounter--;
63        final int currentPriority = getThreadPriority(myTid());
64        if (state.regionCounter == 0 && state.prevPriority != currentPriority) {
65            setThreadPriority(myTid(), state.prevPriority);
66        }
67    }
68
69    /**
70     * Updates the priority we boost the threads to, and updates the current thread's priority if
71     * necessary.
72     */
73    protected void setBoostToPriority(int priority) {
74
75        // We don't care about the other threads here, as long as they see the update of this
76        // variable immediately.
77        mBoostToPriority = priority;
78        final PriorityState state = mThreadState.get();
79        final int tid = myTid();
80        final int prevPriority = getThreadPriority(tid);
81        if (state.regionCounter != 0 && prevPriority != priority) {
82            setThreadPriority(tid, priority);
83        }
84    }
85
86    private static class PriorityState {
87
88        /**
89         * Acts as counter for number of synchronized region that needs to acquire 'this' as a lock
90         * the current thread is currently in. When it drops down to zero, we will no longer boost
91         * the thread's priority.
92         */
93        int regionCounter;
94
95        /**
96         * The thread's previous priority before boosting.
97         */
98        int prevPriority;
99    }
100}