Add VmStat atom and read the count of OOM kills

We want to understand how frequently the OOM killer is invoked in our
droidfood / prod fleets. To do so, introduce a /proc/vmstat reading atom
which we can subsequently use as a ValueMetric to count number of OOM
kills per session.

Bug: 154233512
Test: atest VmStatTests
Change-Id: I58ad126aa691c4ee1c61468e4c5ff5331649791c

services/core/java/com/android/server/stats/pull/ProcfsMemoryUtil.java

@@ -30,6 +30,9 @@ public final class ProcfsMemoryUtil {
             "RssAnon:",
             "VmSwap:"
     };
+    private static final String[] VMSTAT_KEYS = new String[] {
+            "oom_kill"
+    };
 
     private ProcfsMemoryUtil() {}
 
@@ -99,4 +102,22 @@ public final class ProcfsMemoryUtil {
         public int anonRssInKilobytes;
         public int swapInKilobytes;
     }
+
+    /** Reads and parses selected entries of /proc/vmstat. */
+    @Nullable
+    static VmStat readVmStat() {
+        long[] vmstat = new long[VMSTAT_KEYS.length];
+        vmstat[0] = -1;
+        Process.readProcLines("/proc/vmstat", VMSTAT_KEYS, vmstat);
+        if (vmstat[0] == -1) {
+            return null;
+        }
+        VmStat result = new VmStat();
+        result.oomKillCount = (int) vmstat[0];
+        return result;
+    }
+
+    static final class VmStat {
+        public int oomKillCount;
+    }
 }

services/core/java/com/android/server/stats/pull/StatsPullAtomService.java

@@ -544,6 +544,8 @@ public class StatsPullAtomService extends SystemService {
                         return pullProcessDmabufMemory(atomTag, data);
                     case FrameworkStatsLog.SYSTEM_MEMORY:
                         return pullSystemMemory(atomTag, data);
+                    case FrameworkStatsLog.VMSTAT:
+                        return pullVmStat(atomTag, data);
                     case FrameworkStatsLog.TEMPERATURE:
                         synchronized (mTemperatureLock) {
                             return pullTemperatureLocked(atomTag, data);
@@ -842,6 +844,7 @@ public class StatsPullAtomService extends SystemService {
         registerProcessSystemIonHeapSize();
         registerSystemMemory();
         registerProcessDmabufMemory();
+        registerVmStat();
         registerTemperature();
         registerCoolingDevice();
         registerBinderCallsStats();
@@ -2273,6 +2276,27 @@ public class StatsPullAtomService extends SystemService {
         return StatsManager.PULL_SUCCESS;
     }
 
+    private void registerVmStat() {
+        int tagId = FrameworkStatsLog.VMSTAT;
+        mStatsManager.setPullAtomCallback(
+                tagId,
+                null, // use default PullAtomMetadata values
+                DIRECT_EXECUTOR,
+                mStatsCallbackImpl
+        );
+    }
+
+    int pullVmStat(int atomTag, List<StatsEvent> pulledData) {
+        ProcfsMemoryUtil.VmStat vmStat = ProcfsMemoryUtil.readVmStat();
+        if (vmStat != null) {
+            pulledData.add(
+                    FrameworkStatsLog.buildStatsEvent(
+                            atomTag,
+                            vmStat.oomKillCount));
+        }
+        return StatsManager.PULL_SUCCESS;
+    }
+
     private void registerTemperature() {
         int tagId = FrameworkStatsLog.TEMPERATURE;
         mStatsManager.setPullAtomCallback(