bitbake: build: Add BB_TASK_NICE_LEVEL to task code

On Linux its not possible for processes to regain a previous nice level after
it has changed. Its therefore not possible to have a core low priority and
then raise the priorities of individual tasks.

This variable allows us to do something like:

BB_TASK_NICE_LEVEL = "5"
BB_TASK_NICE_LEVEL_task-testimage = "0"

to give priority to specific tasks which the BB_NICE_LEVEL functionality
doesn't give us the option of.

(Bitbake rev: 94d82997220c6cfc7028f76719df028ba8254a5c)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/build.py

@@ -346,6 +346,14 @@ def _exec_task(fn, task, d, quieterr):
     if not tempdir:
         bb.fatal("T variable not set, unable to build")
 
+    # Change nice level if we're asked to
+    nice = localdata.getVar("BB_TASK_NICE_LEVEL", True)
+    if nice:
+        curnice = os.nice(0)
+        nice = int(nice) - curnice
+        newnice = os.nice(nice)
+        logger.debug(1, "Renice to %s " % newnice)
+
     bb.utils.mkdirhier(tempdir)
 
     # Determine the logfile to generate