[tools] Allow specifying cores for cpu.sh script.

BUG=

Review URL: https://codereview.chromium.org/1458833004

Cr-Commit-Position: refs/heads/master@{#32077}

tools/cpu.sh

@@ -14,26 +14,38 @@ set_governor() {
   done
 }
 
-dual_core() {
-  echo "Switching to dual-core mode"
-  for (( i=2; i<=$MAXID; i++ )); do
+enable_cores() {
+  # $1: How many cores to enable.
+  for (( i=1; i<=$MAXID; i++ )); do
+    if [ "$i" -lt "$1" ]; then
+      echo 1 > $CPUPATH/cpu$i/online
+    else
       echo 0 > $CPUPATH/cpu$i/online
+    fi
   done
 }
 
+dual_core() {
+  echo "Switching to dual-core mode"
+  enable_cores 2
+}
+
 single_core() {
   echo "Switching to single-core mode"
-  for (( i=1; i<=$MAXID; i++ )); do
-    echo 0 > $CPUPATH/cpu$i/online
-  done
+  enable_cores 1
 }
 
 
 all_cores() {
   echo "Reactivating all CPU cores"
-  for (( i=1; i<=$MAXID; i++ )); do
-    echo 1 > $CPUPATH/cpu$i/online
-  done
+  enable_cores $((MAXID+1))
+}
+
+
+limit_cores() {
+  # $1: How many cores to enable.
+  echo "Limiting to $1 cores"
+  enable_cores $1
 }
 
 case "$1" in
@@ -55,8 +67,15 @@ case "$1" in
   allcores | all)
     all_cores
     ;;
+  limit_cores)
+    if [ $# -ne 2 ]; then
+      echo "Usage $0 limit_cores <num>"
+      exit 1
+    fi
+    limit_cores $2
+    ;;
   *)
-    echo "Usage: $0 fast|slow|default|singlecore|dualcore|all"
+    echo "Usage: $0 fast|slow|default|singlecore|dualcore|all|limit_cores"
     exit 1
     ;;
 esac