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