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      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 
     17 package com.android.server;
     18 
     19 import static android.os.Process.getThreadPriority;
     20 import static android.os.Process.myTid;
     21 import static android.os.Process.setThreadPriority;
     22 
     23 /**
     24  * Utility class to boost threads in sections where important locks are held.
     25  */
     26 public 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 }