Monitor CPU availability using CPU frequency stats.

- Implement the core CPU availability monitoring logic by reading
  the CPU infos with CpuInfoReader.
- Start debug monitoring on user-debug and eng builds when no
  callbacks are registered.
- Update CpuAvailabilityInfo to cache the uptime of the cached data.
- Update CpuAvailabilityMonitoringConfig to return cpuset string.

Test: atest CpuMonitorServiceTest
Bug: 242722241
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1a18fd0962728468830cc2f2752c1ede3e0c1473)
Merged-In: I6e12c11f243966914079f54df4430d69b797ec1f
Change-Id: I6e12c11f243966914079f54df4430d69b797ec1f

services/core/java/com/android/server/cpu/CpuAvailabilityInfo.java

@@ -21,6 +21,8 @@ import static com.android.server.cpu.CpuAvailabilityMonitoringConfig.CPUSET_BACK
 
 import com.android.internal.util.Preconditions;
 
+import java.util.Objects;
+
 /** CPU availability information. */
 public final class CpuAvailabilityInfo {
     /** Constant to indicate missing CPU availability percent. */
@@ -35,29 +37,64 @@ public final class CpuAvailabilityInfo {
     @CpuAvailabilityMonitoringConfig.Cpuset
     public final int cpuset;
 
+    /** Uptime (in milliseconds) when the data in this object was captured. */
+    public final long dataTimestampUptimeMillis;
+
     /** The latest average CPU availability percent. */
     public final int latestAvgAvailabilityPercent;
 
-    /** The past N-second average CPU availability percent. */
-    public final int pastNSecAvgAvailabilityPercent;
+    /**
+     * The past N-millisecond average CPU availability percent.
+     *
+     * <p>When there is not enough data to calculate the past N-millisecond average, this field will
+     * contain the value {@link MISSING_CPU_AVAILABILITY_PERCENT}.
+     */
+    public final int pastNMillisAvgAvailabilityPercent;
 
-    /** The duration over which the {@link pastNSecAvgAvailabilityPercent} was calculated. */
-    public final int avgAvailabilityDurationSec;
+    /** The duration over which the {@link pastNMillisAvgAvailabilityPercent} was calculated. */
+    public final long pastNMillisDuration;
 
     @Override
     public String toString() {
-        return "CpuAvailabilityInfo{" + "cpuset=" + cpuset + ", latestAvgAvailabilityPercent="
-                + latestAvgAvailabilityPercent + ", pastNSecAvgAvailabilityPercent="
-                + pastNSecAvgAvailabilityPercent + ", avgAvailabilityDurationSec="
-                + avgAvailabilityDurationSec + '}';
+        return "CpuAvailabilityInfo{" + "cpuset = " + cpuset + ", dataTimestampUptimeMillis = "
+                + dataTimestampUptimeMillis + ", latestAvgAvailabilityPercent = "
+                + latestAvgAvailabilityPercent + ", pastNMillisAvgAvailabilityPercent = "
+                + pastNMillisAvgAvailabilityPercent + ", pastNMillisDuration = "
+                + pastNMillisDuration + '}';
     }
 
-    CpuAvailabilityInfo(int cpuset, int latestAvgAvailabilityPercent,
-            int pastNSecAvgAvailabilityPercent, int avgAvailabilityDurationSec) {
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) {
+            return true;
+        }
+        if (!(obj instanceof CpuAvailabilityInfo)) {
+            return false;
+        }
+        CpuAvailabilityInfo info = (CpuAvailabilityInfo) obj;
+        return cpuset == info.cpuset && dataTimestampUptimeMillis == info.dataTimestampUptimeMillis
+                && latestAvgAvailabilityPercent == info.latestAvgAvailabilityPercent
+                && pastNMillisAvgAvailabilityPercent == info.pastNMillisAvgAvailabilityPercent
+                && pastNMillisDuration == info.pastNMillisDuration;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(cpuset, dataTimestampUptimeMillis, latestAvgAvailabilityPercent,
+                pastNMillisAvgAvailabilityPercent, pastNMillisDuration);
+    }
+
+    CpuAvailabilityInfo(int cpuset, long dataTimestampUptimeMillis,
+            int latestAvgAvailabilityPercent, int pastNMillisAvgAvailabilityPercent,
+            long pastNMillisDuration) {
         this.cpuset = Preconditions.checkArgumentInRange(cpuset, CPUSET_ALL, CPUSET_BACKGROUND,
                 "cpuset");
-        this.latestAvgAvailabilityPercent = latestAvgAvailabilityPercent;
-        this.pastNSecAvgAvailabilityPercent = pastNSecAvgAvailabilityPercent;
-        this.avgAvailabilityDurationSec = avgAvailabilityDurationSec;
+        this.dataTimestampUptimeMillis =
+                Preconditions.checkArgumentNonnegative(dataTimestampUptimeMillis);
+        this.latestAvgAvailabilityPercent = Preconditions.checkArgumentNonnegative(
+                latestAvgAvailabilityPercent);
+        this.pastNMillisAvgAvailabilityPercent = pastNMillisAvgAvailabilityPercent;
+        this.pastNMillisDuration = Preconditions.checkArgumentNonnegative(
+                pastNMillisDuration);
     }
 }

services/core/java/com/android/server/cpu/CpuAvailabilityMonitoringConfig.java

@@ -90,8 +90,19 @@ public final class CpuAvailabilityMonitoringConfig {
 
     @Override
     public String toString() {
-        return "CpuAvailabilityMonitoringConfig{cpuset=" + cpuset + ", mThresholds=" + mThresholds
-                + ')';
+        return "CpuAvailabilityMonitoringConfig{cpuset=" + toCpusetString(cpuset) + ", mThresholds="
+                + mThresholds + ')';
+    }
+
+    /** Returns the string equivalent of the provided cpuset. */
+    public static String toCpusetString(int cpuset) {
+        switch (cpuset) {
+            case CPUSET_ALL:
+                return "CPUSET_ALL";
+            case CPUSET_BACKGROUND:
+                return "CPUSET_BACKGROUND";
+        }
+        return "Invalid cpuset: " + cpuset;
     }
 
     private CpuAvailabilityMonitoringConfig(Builder builder) {

services/core/java/com/android/server/cpu/CpuMonitorService.java

@@ -18,15 +18,31 @@ package com.android.server.cpu;
 
 import static android.os.IServiceManager.DUMP_FLAG_PRIORITY_CRITICAL;
 
+import static com.android.server.cpu.CpuAvailabilityMonitoringConfig.CPUSET_ALL;
+import static com.android.server.cpu.CpuAvailabilityMonitoringConfig.CPUSET_BACKGROUND;
+import static com.android.server.cpu.CpuInfoReader.FLAG_CPUSET_CATEGORY_BACKGROUND;
+import static com.android.server.cpu.CpuInfoReader.FLAG_CPUSET_CATEGORY_TOP_APP;
+
+import android.annotation.Nullable;
 import android.content.Context;
 import android.os.Binder;
-import android.util.ArrayMap;
+import android.os.Build;
+import android.os.Handler;
+import android.os.HandlerThread;
+import android.os.Process;
+import android.os.SystemClock;
 import android.util.IndentingPrintWriter;
 import android.util.Log;
+import android.util.LongSparseArray;
+import android.util.SparseArray;
+import android.util.SparseArrayMap;
 
 import com.android.internal.annotations.GuardedBy;
+import com.android.internal.annotations.VisibleForTesting;
 import com.android.internal.util.DumpUtils;
+import com.android.server.ServiceThread;
 import com.android.server.SystemService;
+import com.android.server.Watchdog;
 import com.android.server.utils.PriorityDump;
 import com.android.server.utils.Slogf;
 
@@ -34,28 +50,52 @@ import java.io.FileDescriptor;
 import java.io.PrintWriter;
 import java.util.Objects;
 import java.util.concurrent.Executor;
+import java.util.concurrent.TimeUnit;
 
 /** Service to monitor CPU availability and usage. */
 public final class CpuMonitorService extends SystemService {
     static final String TAG = CpuMonitorService.class.getSimpleName();
-    static final boolean DEBUG = Log.isLoggable(TAG, Log.DEBUG);
-    // TODO(b/242722241): Make this a resource overlay property.
-    //  Maintain 3 monitoring intervals:
-    //  * One to poll very frequently when mCpuAvailabilityCallbackInfoByCallbacks are available and
-    //    CPU availability is above a threshold (such as at least 10% of CPU is available).
-    //  * One to poll less frequently when mCpuAvailabilityCallbackInfoByCallbacks are available
-    //    and CPU availability is below a threshold (such as less than 10% of CPU is available).
-    //  * One to poll very less frequently when no callbacks are available and the build is either
-    //    user-debug or eng. This will be useful for debugging in development environment.
-    static final int DEFAULT_CPU_MONITORING_INTERVAL_MILLISECONDS = 5_000;
+    static final boolean DEBUG =  Log.isLoggable(TAG, Log.DEBUG);
+    // TODO(b/267500110): Make these constants resource overlay properties.
+    /** Default monitoring interval when no monitoring is in progress. */
+    static final long DEFAULT_MONITORING_INTERVAL_MILLISECONDS = -1;
+    // TODO(b/242722241): Add a constant for normal monitoring interval when callbacks are
+    //  registered.
+    /**
+     * Monitoring interval when no registered callbacks and the build is either user-debug or eng.
+     */
+    private static final long DEBUG_MONITORING_INTERVAL_MILLISECONDS = TimeUnit.MINUTES.toMillis(1);
+    /**
+     * Size of the in-memory cache relative to the current uptime.
+     *
+     * On user-debug or eng builds, continuously cache stats with a bigger cache size for debugging
+     * purposes.
+     */
+    private static final long CACHE_DURATION_MILLISECONDS = Build.IS_USERDEBUG || Build.IS_ENG
+            ? TimeUnit.MINUTES.toMillis(30) : TimeUnit.MINUTES.toMillis(10);
+    // TODO(b/267500110): Investigate whether this duration should change when the monitoring
+    //  interval is updated. When the CPU is under heavy load, the monitoring will happen less
+    //  frequently. Should this duration be increased as well when this happens?
+    private static final long LATEST_AVAILABILITY_DURATION_MILLISECONDS =
+            TimeUnit.SECONDS.toMillis(30);
 
     private final Context mContext;
+    private final HandlerThread mHandlerThread;
+    private final CpuInfoReader mCpuInfoReader;
+    private final boolean mShouldDebugMonitor;
+    private final long mDebugMonitoringIntervalMillis;
+    private final long mLatestAvailabilityDurationMillis;
     private final Object mLock = new Object();
     @GuardedBy("mLock")
-    private final ArrayMap<CpuMonitorInternal.CpuAvailabilityCallback, CpuAvailabilityCallbackInfo>
-            mCpuAvailabilityCallbackInfoByCallbacks = new ArrayMap<>();
+    private final SparseArrayMap<CpuMonitorInternal.CpuAvailabilityCallback,
+            CpuAvailabilityCallbackInfo> mAvailabilityCallbackInfosByCallbacksByCpuset;
     @GuardedBy("mLock")
-    private long mMonitoringIntervalMilliseconds = DEFAULT_CPU_MONITORING_INTERVAL_MILLISECONDS;
+    private final SparseArray<CpusetInfo> mCpusetInfosByCpuset;
+    private final Runnable mMonitorCpuStats = this::monitorCpuStats;
+
+    @GuardedBy("mLock")
+    private long mCurrentMonitoringIntervalMillis = DEFAULT_MONITORING_INTERVAL_MILLISECONDS;
+    private Handler mHandler;
 
     private final CpuMonitorInternal mLocalService = new CpuMonitorInternal() {
         @Override
@@ -64,17 +104,22 @@ public final class CpuMonitorService extends SystemService {
             Objects.requireNonNull(callback, "Callback must be non-null");
             Objects.requireNonNull(config, "Config must be non-null");
             synchronized (mLock) {
-                if (mCpuAvailabilityCallbackInfoByCallbacks.containsKey(callback)) {
-                    Slogf.i(TAG, "Overwriting the existing CpuAvailabilityCallback %s",
-                            mCpuAvailabilityCallbackInfoByCallbacks.get(callback));
-                    // TODO(b/242722241): Overwrite any internal cache (will be added in future CLs)
-                    //  that maps callbacks based on the CPU availability thresholds.
+                // Verify all CPUSET entries before adding the callback because this will help
+                // delete any previously added callback for a different CPUSET.
+                for (int i = 0; i < mAvailabilityCallbackInfosByCallbacksByCpuset.numMaps(); i++) {
+                    int cpuset = mAvailabilityCallbackInfosByCallbacksByCpuset.keyAt(i);
+                    CpuAvailabilityCallbackInfo callbackInfo =
+                            mAvailabilityCallbackInfosByCallbacksByCpuset.delete(cpuset, callback);
+                    if (callbackInfo != null) {
+                        Slogf.i(TAG, "Overwriting the existing %s", callbackInfo);
+                    }
                 }
-                CpuAvailabilityCallbackInfo info = new CpuAvailabilityCallbackInfo(config,
+                CpuAvailabilityCallbackInfo callbackInfo = new CpuAvailabilityCallbackInfo(config,
                         executor);
-                mCpuAvailabilityCallbackInfoByCallbacks.put(callback, info);
+                mAvailabilityCallbackInfosByCallbacksByCpuset.add(config.cpuset, callback,
+                        callbackInfo);
                 if (DEBUG) {
-                    Slogf.d(TAG, "Added a CPU availability callback: %s", info);
+                    Slogf.d(TAG, "Added a CPU availability callback: %s", callbackInfo);
                 }
             }
             // TODO(b/242722241):
@@ -86,58 +131,188 @@ public final class CpuMonitorService extends SystemService {
         @Override
         public void removeCpuAvailabilityCallback(CpuAvailabilityCallback callback) {
             synchronized (mLock) {
-                if (!mCpuAvailabilityCallbackInfoByCallbacks.containsKey(callback)) {
-                    Slogf.i(TAG, "CpuAvailabilityCallback was not previously added."
-                            + " Ignoring the remove request");
-                    return;
-                }
-                CpuAvailabilityCallbackInfo info =
-                        mCpuAvailabilityCallbackInfoByCallbacks.remove(callback);
-                if (DEBUG) {
-                    Slogf.d(TAG, "Removed a CPU availability callback: %s", info);
+                for (int i = 0; i < mAvailabilityCallbackInfosByCallbacksByCpuset.numMaps(); i++) {
+                    int cpuset = mAvailabilityCallbackInfosByCallbacksByCpuset.keyAt(i);
+                    CpuAvailabilityCallbackInfo callbackInfo =
+                            mAvailabilityCallbackInfosByCallbacksByCpuset.delete(cpuset, callback);
+                    if (callbackInfo != null) {
+                        if (DEBUG) {
+                            Slogf.d(TAG, "Successfully removed %s", callbackInfo);
+                        }
+                        return;
+                    }
                 }
+                Slogf.w(TAG, "CpuAvailabilityCallback was not previously added. Ignoring the remove"
+                        + " request");
             }
-            // TODO(b/242722241): Increase CPU monitoring interval when all callbacks are removed.
         }
     };
 
     public CpuMonitorService(Context context) {
+        this(context, new CpuInfoReader(), new ServiceThread(TAG,
+                Process.THREAD_PRIORITY_BACKGROUND, /* allowIo= */ true),
+                Build.IS_USERDEBUG || Build.IS_ENG, DEBUG_MONITORING_INTERVAL_MILLISECONDS,
+                LATEST_AVAILABILITY_DURATION_MILLISECONDS);
+    }
+
+    @VisibleForTesting
+    CpuMonitorService(Context context, CpuInfoReader cpuInfoReader, HandlerThread handlerThread,
+            boolean shouldDebugMonitor, long debugMonitoringIntervalMillis,
+            long latestAvailabilityDurationMillis) {
         super(context);
         mContext = context;
+        mHandlerThread = handlerThread;
+        mShouldDebugMonitor = shouldDebugMonitor;
+        mDebugMonitoringIntervalMillis = debugMonitoringIntervalMillis;
+        mLatestAvailabilityDurationMillis = latestAvailabilityDurationMillis;
+        mCpuInfoReader = cpuInfoReader;
+        mCpusetInfosByCpuset = new SparseArray<>(2);
+        mCpusetInfosByCpuset.append(CPUSET_ALL, new CpusetInfo(CPUSET_ALL));
+        mCpusetInfosByCpuset.append(CPUSET_BACKGROUND, new CpusetInfo(CPUSET_BACKGROUND));
+        mAvailabilityCallbackInfosByCallbacksByCpuset = new SparseArrayMap<>();
     }
 
     @Override
     public void onStart() {
+        // Initialize CPU info reader and perform the first read to make sure the CPU stats are
+        // readable without any issues.
+        if (!mCpuInfoReader.init() || mCpuInfoReader.readCpuInfos() == null) {
+            Slogf.wtf(TAG, "Failed to initialize CPU info reader. This happens when the CPU "
+                    + "frequency stats are not available or the sysfs interface has changed in "
+                    + "the Kernel. Cannot monitor CPU without these stats. Terminating CPU monitor "
+                    + "service");
+            return;
+        }
+        mHandlerThread.start();
+        mHandler = new Handler(mHandlerThread.getLooper());
         publishLocalService(CpuMonitorInternal.class, mLocalService);
         publishBinderService("cpu_monitor", new CpuMonitorBinder(), /* allowIsolated= */ false,
                 DUMP_FLAG_PRIORITY_CRITICAL);
+        Watchdog.getInstance().addThread(mHandler);
+        synchronized (mLock) {
+            if (mShouldDebugMonitor && !mHandler.hasCallbacks(mMonitorCpuStats)) {
+                mCurrentMonitoringIntervalMillis = mDebugMonitoringIntervalMillis;
+                Slogf.i(TAG, "Starting debug monitoring");
+                mHandler.post(mMonitorCpuStats);
+            }
+        }
     }
 
     private void doDump(IndentingPrintWriter writer) {
         writer.printf("*%s*\n", getClass().getSimpleName());
         writer.increaseIndent();
+        mCpuInfoReader.dump(writer);
         synchronized (mLock) {
-            writer.printf("CPU monitoring interval: %d ms\n", mMonitoringIntervalMilliseconds);
-            if (!mCpuAvailabilityCallbackInfoByCallbacks.isEmpty()) {
+            writer.printf("Current CPU monitoring interval: %d ms\n",
+                    mCurrentMonitoringIntervalMillis);
+            if (hasClientCallbacksLocked()) {
                 writer.println("CPU availability change callbacks:");
                 writer.increaseIndent();
-                for (int i = 0; i < mCpuAvailabilityCallbackInfoByCallbacks.size(); i++) {
-                    writer.printf("%s: %s\n", mCpuAvailabilityCallbackInfoByCallbacks.keyAt(i),
-                            mCpuAvailabilityCallbackInfoByCallbacks.valueAt(i));
+                mAvailabilityCallbackInfosByCallbacksByCpuset.forEach(
+                        (callbackInfo) -> writer.printf("%s\n", callbackInfo));
+                writer.decreaseIndent();
+            }
+            if (mCpusetInfosByCpuset.size() > 0) {
+                writer.println("Cpuset infos:");
+                writer.increaseIndent();
+                for (int i = 0; i < mCpusetInfosByCpuset.size(); i++) {
+                    writer.printf("%s\n", mCpusetInfosByCpuset.valueAt(i));
                 }
                 writer.decreaseIndent();
             }
         }
-        // TODO(b/242722241): Print the recent past CPU stats.
         writer.decreaseIndent();
     }
 
+    private void monitorCpuStats() {
+        long uptimeMillis = SystemClock.uptimeMillis();
+        SparseArray<CpuInfoReader.CpuInfo> cpuInfosByCoreId = mCpuInfoReader.readCpuInfos();
+        if (cpuInfosByCoreId == null) {
+            // This shouldn't happen because the CPU infos are read & verified during
+            // the {@link onStart} call.
+            Slogf.wtf(TAG, "Failed to read CPU info from device");
+            synchronized (mLock) {
+                stopMonitoringCpuStatsLocked();
+            }
+            // Monitoring is stopped but no client callback is removed.
+            // TODO(b/267500110): Identify whether the clients should be notified about this state.
+            return;
+        }
+
+        synchronized (mLock) {
+            // 1. Populate the {@link mCpusetInfosByCpuset} with the latest cpuInfo.
+            for (int i = 0; i < cpuInfosByCoreId.size(); i++) {
+                CpuInfoReader.CpuInfo cpuInfo = cpuInfosByCoreId.valueAt(i);
+                for (int j = 0; j < mCpusetInfosByCpuset.size(); j++) {
+                    mCpusetInfosByCpuset.valueAt(j).appendCpuInfo(uptimeMillis, cpuInfo);
+                }
+            }
+
+            // 2. Verify whether any monitoring thresholds are crossed and notify the corresponding
+            // clients.
+            for (int i = 0; i < mCpusetInfosByCpuset.size(); i++) {
+                CpusetInfo cpusetInfo = mCpusetInfosByCpuset.valueAt(i);
+                cpusetInfo.populateLatestCpuAvailabilityInfo(uptimeMillis,
+                        mLatestAvailabilityDurationMillis);
+                // TODO(b/242722241): Check CPU availability against thresholds and notify clients.
+            }
+
+            // TODO(b/267500110): Detect heavy CPU load. On detecting heavy CPU load, increase
+            //  the monitoring interval and notify the clients.
+
+            // 3. Continue monitoring only when either there is at least one registered client
+            // callback or debug monitoring is enabled.
+            if (mCurrentMonitoringIntervalMillis > 0
+                    && (hasClientCallbacksLocked() || mShouldDebugMonitor)) {
+                mHandler.postAtTime(mMonitorCpuStats,
+                        uptimeMillis + mCurrentMonitoringIntervalMillis);
+            } else {
+                stopMonitoringCpuStatsLocked();
+            }
+        }
+    }
+
+    @GuardedBy("mLock")
+    private boolean hasClientCallbacksLocked() {
+        for (int i = 0; i < mAvailabilityCallbackInfosByCallbacksByCpuset.numMaps(); i++) {
+            if (mAvailabilityCallbackInfosByCallbacksByCpuset.numElementsForKeyAt(i) > 0) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    @GuardedBy("mLock")
+    private void stopMonitoringCpuStatsLocked() {
+        mHandler.removeCallbacks(mMonitorCpuStats);
+        mCurrentMonitoringIntervalMillis = DEFAULT_MONITORING_INTERVAL_MILLISECONDS;
+        // When the monitoring is stopped, the latest CPU availability info and the snapshots in
+        // {@code mCpusetInfosByCpuset} will become obsolete soon. So, remove them.
+        for (int i = 0; i < mCpusetInfosByCpuset.size(); i++) {
+            mCpusetInfosByCpuset.valueAt(i).clear();
+        }
+    }
+
+    private static boolean containsCpuset(@CpuInfoReader.CpusetCategory int cpusetCategories,
+            @CpuAvailabilityMonitoringConfig.Cpuset int expectedCpuset) {
+        switch (expectedCpuset) {
+            case CPUSET_ALL:
+                return (cpusetCategories & FLAG_CPUSET_CATEGORY_TOP_APP) != 0;
+            case CPUSET_BACKGROUND:
+                return (cpusetCategories & FLAG_CPUSET_CATEGORY_BACKGROUND) != 0;
+            default:
+                Slogf.wtf(TAG, "Provided invalid expectedCpuset %d", expectedCpuset);
+        }
+        return false;
+    }
+
     private static final class CpuAvailabilityCallbackInfo {
         public final CpuAvailabilityMonitoringConfig config;
+        @Nullable
         public final Executor executor;
 
         CpuAvailabilityCallbackInfo(CpuAvailabilityMonitoringConfig config,
-                Executor executor) {
+                @Nullable Executor executor) {
             this.config = config;
             this.executor = executor;
         }
@@ -170,4 +345,144 @@ public final class CpuMonitorService extends SystemService {
             PriorityDump.dump(mPriorityDumper, fd, pw, args);
         }
     }
+
+    private static final class CpusetInfo {
+        @CpuAvailabilityMonitoringConfig.Cpuset
+        public final int cpuset;
+        private final LongSparseArray<Snapshot> mSnapshotsByUptime;
+        @Nullable
+        private CpuAvailabilityInfo mLatestCpuAvailabilityInfo;
+
+        CpusetInfo(int cpuset) {
+            this.cpuset = cpuset;
+            mSnapshotsByUptime = new LongSparseArray<>();
+        }
+
+        public void appendCpuInfo(long uptimeMillis, CpuInfoReader.CpuInfo cpuInfo) {
+            if (!containsCpuset(cpuInfo.cpusetCategories, cpuset)) {
+                return;
+            }
+            Snapshot currentSnapshot = mSnapshotsByUptime.get(uptimeMillis);
+            if (currentSnapshot == null) {
+                currentSnapshot = new Snapshot(uptimeMillis);
+                mSnapshotsByUptime.append(uptimeMillis, currentSnapshot);
+                if (mSnapshotsByUptime.size() > 0
+                        && (uptimeMillis - mSnapshotsByUptime.valueAt(0).uptimeMillis)
+                        > CACHE_DURATION_MILLISECONDS) {
+                    mSnapshotsByUptime.removeAt(0);
+                }
+            }
+            currentSnapshot.appendCpuInfo(cpuInfo);
+        }
+
+        public void populateLatestCpuAvailabilityInfo(long currentUptimeMillis,
+                long latestAvailabilityDurationMillis) {
+            int numSnapshots = mSnapshotsByUptime.size();
+            if (numSnapshots == 0) {
+                mLatestCpuAvailabilityInfo = null;
+                return;
+            }
+            Snapshot latestSnapshot = mSnapshotsByUptime.valueAt(numSnapshots - 1);
+            if (latestSnapshot.uptimeMillis != currentUptimeMillis) {
+                // When the cpuset has no stats available for the current polling, the uptime will
+                // mismatch. When this happens, return {@code null} to avoid returning stale
+                // information.
+                if (DEBUG) {
+                    Slogf.d(TAG, "Skipping stale CPU availability information for cpuset %s",
+                            CpuAvailabilityMonitoringConfig.toCpusetString(cpuset));
+                }
+                mLatestCpuAvailabilityInfo = null;
+                return;
+            }
+            // Avoid constructing {@link mLatestCpuAvailabilityInfo} if the uptime hasn't changed.
+            if (mLatestCpuAvailabilityInfo != null
+                    && mLatestCpuAvailabilityInfo.dataTimestampUptimeMillis
+                    == latestSnapshot.uptimeMillis) {
+                return;
+            }
+            long earliestUptimeMillis = currentUptimeMillis - latestAvailabilityDurationMillis;
+            mLatestCpuAvailabilityInfo = new CpuAvailabilityInfo(cpuset,
+                    latestSnapshot.uptimeMillis, latestSnapshot.getAverageAvailableCpuFreqPercent(),
+                    getCumulativeAvgAvailabilityPercent(earliestUptimeMillis),
+                    latestAvailabilityDurationMillis);
+        }
+
+        private int getCumulativeAvgAvailabilityPercent(long earliestUptimeMillis) {
+            long totalAvailableCpuFreqKHz = 0;
+            long totalOnlineMaxCpuFreqKHz = 0;
+            int totalAccountedSnapshots = 0;
+            long earliestSeenUptimeMillis = Long.MAX_VALUE;
+            for (int i = mSnapshotsByUptime.size() - 1; i >= 0; i--) {
+                Snapshot snapshot = mSnapshotsByUptime.valueAt(i);
+                earliestSeenUptimeMillis = snapshot.uptimeMillis;
+                if (snapshot.uptimeMillis <= earliestUptimeMillis) {
+                    break;
+                }
+                totalAccountedSnapshots++;
+                totalAvailableCpuFreqKHz += snapshot.totalNormalizedAvailableCpuFreqKHz;
+                totalOnlineMaxCpuFreqKHz += snapshot.totalOnlineMaxCpuFreqKHz;
+            }
+            // The cache must have at least 2 snapshots within the given duration and
+            // the {@link earliestSeenUptimeMillis} must be earlier than (i,e., less than) the given
+            // {@link earliestUptimeMillis}. Otherwise, the cache doesn't have enough data to
+            // calculate the cumulative average for the given duration.
+            // TODO(b/267500110): Investigate whether the cumulative average duration should be
+            //  shrunk when not enough data points are available.
+            if (earliestSeenUptimeMillis > earliestUptimeMillis || totalAccountedSnapshots < 2) {
+                return CpuAvailabilityInfo.MISSING_CPU_AVAILABILITY_PERCENT;
+            }
+            return (int) ((totalAvailableCpuFreqKHz * 100.0) / totalOnlineMaxCpuFreqKHz);
+        }
+
+        public void clear() {
+            mLatestCpuAvailabilityInfo = null;
+            mSnapshotsByUptime.clear();
+        }
+
+        @Override
+        public String toString() {
+            return "CpusetInfo{cpuset = " + CpuAvailabilityMonitoringConfig.toCpusetString(cpuset)
+                    + ", mSnapshotsByUptime = " + mSnapshotsByUptime
+                    + ", mLatestCpuAvailabilityInfo = " + mLatestCpuAvailabilityInfo + '}';
+        }
+
+        private static final class Snapshot {
+            public final long uptimeMillis;
+            public int totalOnlineCpus;
+            public int totalOfflineCpus;
+            public long totalNormalizedAvailableCpuFreqKHz;
+            public long totalOnlineMaxCpuFreqKHz;
+            public long totalOfflineMaxCpuFreqKHz;
+
+            Snapshot(long uptimeMillis) {
+                this.uptimeMillis = uptimeMillis;
+            }
+
+            public void appendCpuInfo(CpuInfoReader.CpuInfo cpuInfo) {
+                if (!cpuInfo.isOnline) {
+                    totalOfflineCpus++;
+                    totalOfflineMaxCpuFreqKHz += cpuInfo.maxCpuFreqKHz;
+                    return;
+                }
+                ++totalOnlineCpus;
+                totalNormalizedAvailableCpuFreqKHz += cpuInfo.getNormalizedAvailableCpuFreqKHz();
+                totalOnlineMaxCpuFreqKHz += cpuInfo.maxCpuFreqKHz;
+            }
+
+            public int getAverageAvailableCpuFreqPercent() {
+                return (int) ((totalNormalizedAvailableCpuFreqKHz * 100.0)
+                        / totalOnlineMaxCpuFreqKHz);
+            }
+
+            @Override
+            public String toString() {
+                return "Snapshot{uptimeMillis = " + uptimeMillis + ", totalOnlineCpus = "
+                        + totalOnlineCpus + ", totalOfflineCpus = " + totalOfflineCpus
+                        + ", totalNormalizedAvailableCpuFreqKHz = "
+                        + totalNormalizedAvailableCpuFreqKHz
+                        + ", totalOnlineMaxCpuFreqKHz = " + totalOnlineMaxCpuFreqKHz
+                        + ", totalOfflineMaxCpuFreqKHz = " + totalOfflineMaxCpuFreqKHz + '}';
+            }
+        }
+    }
 }