Add new OOM adjustment for the "previous" process.

This is the process that you had previously been interacting with
in the UI before the current one.  Treating it specially should
allow us to improve the scenario of switching back and forth
between two apps.

Also add API constent for ICS MR1.

Change-Id: Ib3fe4df36b270be11dfd6b7e8d107c9994058a4d

3 files changed, 48 insertions, 6 deletions

diff --git a/services/java/com/android/server/am/ActivityManagerService.java b/services/java/com/android/server/am/ActivityManagerService.java
index f8a7d6a..310ace0 100644
--- a/services/java/com/android/server/am/ActivityManagerService.java
+++ b/services/java/com/android/server/am/ActivityManagerService.java
@@ -408,6 +408,12 @@ public final class ActivityManagerService extends ActivityManagerNative
     ProcessRecord mHomeProcess;
     
     /**
+     * This is the process holding the activity the user last visited that
+     * is in a different process from the one they are currently in.
+     */
+    ProcessRecord mPreviousProcess;
+    
+    /**
      * Packages that the user has asked to have run in screen size
      * compatibility mode instead of filling the screen.
      */
@@ -8114,6 +8120,7 @@ public final class ActivityManagerService extends ActivityManagerNative
 
         pw.println();
         pw.println("  mHomeProcess: " + mHomeProcess);
+        pw.println("  mPreviousProcess: " + mPreviousProcess);
         if (mHeavyWeightProcess != null) {
             pw.println("  mHeavyWeightProcess: " + mHeavyWeightProcess);
         }
@@ -8212,6 +8219,7 @@ public final class ActivityManagerService extends ActivityManagerNative
             pw.print("    BACKUP_APP_ADJ: "); pw.println(ProcessList.BACKUP_APP_ADJ);
             pw.print("    SERVICE_ADJ: "); pw.println(ProcessList.SERVICE_ADJ);
             pw.print("    HOME_APP_ADJ: "); pw.println(ProcessList.HOME_APP_ADJ);
+            pw.print("    PREVIOUS_APP_ADJ: "); pw.println(ProcessList.PREVIOUS_APP_ADJ);
             pw.print("    SERVICE_B_ADJ: "); pw.println(ProcessList.SERVICE_B_ADJ);
             pw.print("    HIDDEN_APP_MIN_ADJ: "); pw.println(ProcessList.HIDDEN_APP_MIN_ADJ);
             pw.print("    HIDDEN_APP_MAX_ADJ: "); pw.println(ProcessList.HIDDEN_APP_MAX_ADJ);
@@ -8228,6 +8236,7 @@ public final class ActivityManagerService extends ActivityManagerNative
 
         pw.println();
         pw.println("  mHomeProcess: " + mHomeProcess);
+        pw.println("  mPreviousProcess: " + mPreviousProcess);
         if (mHeavyWeightProcess != null) {
             pw.println("  mHeavyWeightProcess: " + mHeavyWeightProcess);
         }
@@ -9009,6 +9018,8 @@ public final class ActivityManagerService extends ActivityManagerNative
                 oomAdj = buildOomTag("bak", "  ", r.setAdj, ProcessList.HIDDEN_APP_MIN_ADJ);
             } else if (r.setAdj >= ProcessList.SERVICE_B_ADJ) {
                 oomAdj = buildOomTag("svcb ", null, r.setAdj, ProcessList.SERVICE_B_ADJ);
+            } else if (r.setAdj >= ProcessList.PREVIOUS_APP_ADJ) {
+                oomAdj = buildOomTag("prev ", null, r.setAdj, ProcessList.PREVIOUS_APP_ADJ);
             } else if (r.setAdj >= ProcessList.HOME_APP_ADJ) {
                 oomAdj = buildOomTag("home ", null, r.setAdj, ProcessList.HOME_APP_ADJ);
             } else if (r.setAdj >= ProcessList.SERVICE_ADJ) {
@@ -9287,12 +9298,13 @@ public final class ActivityManagerService extends ActivityManagerNative
             ProcessList.SYSTEM_ADJ, ProcessList.PERSISTENT_PROC_ADJ, ProcessList.FOREGROUND_APP_ADJ,
             ProcessList.VISIBLE_APP_ADJ, ProcessList.PERCEPTIBLE_APP_ADJ, ProcessList.HEAVY_WEIGHT_APP_ADJ,
             ProcessList.BACKUP_APP_ADJ, ProcessList.SERVICE_ADJ, ProcessList.HOME_APP_ADJ,
-            ProcessList.SERVICE_B_ADJ, ProcessList.HIDDEN_APP_MAX_ADJ
+            ProcessList.PREVIOUS_APP_ADJ, ProcessList.SERVICE_B_ADJ, ProcessList.HIDDEN_APP_MAX_ADJ
         };
         final String[] oomLabel = new String[] {
                 "System", "Persistent", "Foreground",
                 "Visible", "Perceptible", "Heavy Weight",
-                "Backup", "A Services", "Home", "B Services", "Background"
+                "Backup", "A Services", "Home", "Previous",
+                "B Services", "Background"
         };
         long oomPss[] = new long[oomLabel.length];
         ArrayList<MemItem>[] oomProcs = (ArrayList<MemItem>[])new ArrayList[oomLabel.length];
@@ -9714,7 +9726,10 @@ public final class ActivityManagerService extends ActivityManagerNative
         if (app == mHomeProcess) {
             mHomeProcess = null;
         }
-        
+        if (app == mPreviousProcess) {
+            mPreviousProcess = null;
+        }
+
         if (restart) {
             // We have components that still need to be running in the
             // process, so re-launch it.
@@ -13112,6 +13127,17 @@ public final class ActivityManagerService extends ActivityManagerNative
             app.adjType = "home";
         }
 
+        if (adj > ProcessList.PREVIOUS_APP_ADJ && app == mPreviousProcess
+                && app.activities.size() > 0) {
+            // This was the previous process that showed UI to the user.
+            // We want to try to keep it around more aggressively, to give
+            // a good experience around switching between two apps.
+            adj = ProcessList.PREVIOUS_APP_ADJ;
+            schedGroup = Process.THREAD_GROUP_BG_NONINTERACTIVE;
+            app.hidden = false;
+            app.adjType = "previous";
+        }
+
         if (false) Slog.i(TAG, "OOM " + app + ": initial adj=" + adj
                 + " reason=" + app.adjType);
 
@@ -13841,7 +13867,8 @@ public final class ActivityManagerService extends ActivityManagerNative
                     } else {
                         numBg++;
                     }
-                } else if (app.curAdj >= ProcessList.HOME_APP_ADJ) {
+                } else if (app.curAdj >= ProcessList.HOME_APP_ADJ
+                        && app.curAdj != ProcessList.SERVICE_B_ADJ) {
                     numBg++;
                 }
             }
diff --git a/services/java/com/android/server/am/ActivityStack.java b/services/java/com/android/server/am/ActivityStack.java
index 28c3bae..a47502e 100644
--- a/services/java/com/android/server/am/ActivityStack.java
+++ b/services/java/com/android/server/am/ActivityStack.java
@@ -1363,6 +1363,13 @@ final class ActivityStack {
                         + ", nowVisible=" + next.nowVisible);
                 }
             }
+
+            if (!prev.finishing && prev.app != null && prev.app != next.app) {
+                // We are switching to a new activity that is in a different
+                // process than the previous one.  Note the previous process,
+                // so we can try to keep it around.
+                mService.mPreviousProcess = prev.app;
+            }
         }
 
         // Launching this app's activity, make sure the app is no longer
diff --git a/services/java/com/android/server/am/ProcessList.java b/services/java/com/android/server/am/ProcessList.java
index f368a70..af7b314 100644
--- a/services/java/com/android/server/am/ProcessList.java
+++ b/services/java/com/android/server/am/ProcessList.java
@@ -38,11 +38,19 @@ class ProcessList {
     // This is a process only hosting activities that are not visible,
     // so it can be killed without any disruption.
     static final int HIDDEN_APP_MAX_ADJ = 15;
-    static int HIDDEN_APP_MIN_ADJ = 8;
+    static int HIDDEN_APP_MIN_ADJ = 9;
 
     // The B list of SERVICE_ADJ -- these are the old and decrepit
     // services that aren't as shiny and interesting as the ones in the A list.
-    static final int SERVICE_B_ADJ = 7;
+    static final int SERVICE_B_ADJ = 8;
+
+    // This is the process of the previous application that the user was in.
+    // This process is kept above other things, because it is very common to
+    // switch back to the previous app.  This is important both for recent
+    // task switch (toggling between the two top recent apps) as well as normal
+    // UI flow such as clicking on a URI in the e-mail app to view in the browser,
+    // and then pressing back to return to e-mail.
+    static final int PREVIOUS_APP_ADJ = 7;
 
     // This is a process holding the home application -- we want to try
     // avoiding killing it, even if it would normally be in the background,