Speed up chroot checking loop

It was noticed on a very busy system this can take about 1s per loop.
This starts to add up on thousands of processes.

Firstly, prune out all the kernel threads.  Then introduce a very
small inline python script to find any pids that seem to be in the
chroot without forking to examine each one.  After that the existing
loop just kills anything as before.

Change-Id: Icc7bc7eda80ffcd636f97e6542d70c220e9c225e

lib/common-functions

@@ -135,24 +135,44 @@ function eval_run_d () {
     trap - ERR
 }
 
+# Get any process that appears to be running in $TMP_BUILD_DIR
+function _get_chroot_processes () {
+    # Deselect kernel threads, and use a python script to avoid
+    # forking lots and lots of readlink / grep processes on a busy
+    # system.
+    ps --ppid 2 -p 2 --deselect -o pid= | xargs python -c '
+import os
+import sys
+
+for pid in sys.argv[2:]:
+    try:
+        root = os.readlink("/proc/%s/root" % pid)
+    except:
+        continue
+    if sys.argv[1] in root:
+        print("%s" % pid)
+' $TMP_BUILD_DIR
+}
+
+
 function kill_chroot_processes () {
     local xtrace
     xtrace=$(set +o | grep xtrace)
     set +o xtrace
 
+    local pidname
+
     if [ -z "${1}" ]; then
         echo "ERROR: no chroot directory specified"
         exit 1
     fi
-    for piddir in /proc/[0-9]*; do
-        pid=${piddir##/proc/}
-        pidname=$(cat $piddir/comm 2>/dev/null || echo "unknown")
+
+    for pid in $(_get_chroot_processes); do
         # If there are open files from the chroot, just kill the process using
-        # these files.
-        if sudo readlink -f $piddir/root | grep -q $TMP_BUILD_DIR; then
-            echo "Killing chroot process: '${pidname}($pid)'"
-            sudo kill $pid
-        fi
+        # these files.  This is racy, but good enough
+        pidname=$(cat $piddir/comm 2>/dev/null || echo "unknown")
+        echo "Killing chroot process: '${pidname}($pid)'"
+        sudo kill $pid
     done
 
     $xtrace