trigger kernel blocked stack trace on system server watchdog

Bug b/7638530 may be caused by a kernel deadlock when killing
processes under low memory conditions.  Write to /proc/sysrq-trigger
to get a kernel log of blocked tasks before killing the system server.

Bug: 7638530
Change-Id: I60df324ad4affdadbf13650099dc4dfb38722420

Author: colincross

services/java/com/android/server/Watchdog.java

@@ -39,6 +39,8 @@
 import android.util.Slog;
 
 import java.io.File;
+import java.io.FileWriter;
+import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Calendar;
 
@@ -439,6 +441,16 @@ public void run() {
                 dumpKernelStackTraces();
             }
 
+            // Trigger the kernel to dump all blocked threads to the kernel log
+            try {
+                FileWriter sysrq_trigger = new FileWriter("/proc/sysrq-trigger");
+                sysrq_trigger.write("w");
+                sysrq_trigger.close();
+            } catch (IOException e) {
+                Slog.e(TAG, "Failed to write to /proc/sysrq-trigger");
+                Slog.e(TAG, e.getMessage());
+            }
+
             // Try to add the error to the dropbox, but assuming that the ActivityManager
             // itself may be deadlocked.  (which has happened, causing this statement to
             // deadlock and the watchdog as a whole to be ineffective)