bitbake: runqueue: Add support for BB_LOADFACTOR_MAX

Some ditros don't enable /proc/pressure and it tends to be those which we
see bitbake timeout issues on, seemingly as load gets too high and the bitbake
processes don't get scheduled in for minutes at a time.

Add support for stopping running extra tasks if the system load average goes
above a certain threshold by setting BB_LOADFACTOR_MAX.

The value used is scaled by CPU number, so a value of 1 would be when
the load average equals the number of cpu cores of the system, under one
only starts tasks when the load average is below the number of cores.

This means you can centrally set a value such as 1.5 which will then
scale correctly to different sized machines with differing numbers
of CPUs.

The pressure regulation is probably more accurate and responsive, however
our graphs do show singificant load spikes on some workers and this
patch is aimed at trying to avoid those.

Pressure regulation is used where available in preference to this load
factor regulation when both are set.

(Bitbake rev: 14a27306f6dceb4999c2804ccae5a09cc3d8dd49)

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

bitbake/lib/bb/runqueue.py

@@ -220,6 +220,16 @@ class RunQueueScheduler(object):
                 bb.note("Pressure status changed to CPU: %s, IO: %s, Mem: %s (CPU: %s/%s, IO: %s/%s, Mem: %s/%s) - using %s/%s bitbake threads" % (pressure_state + pressure_values + (len(self.rq.runq_running.difference(self.rq.runq_complete)), self.rq.number_tasks)))
             self.pressure_state = pressure_state
             return (exceeds_cpu_pressure or exceeds_io_pressure or exceeds_memory_pressure)
+        elif self.rq.max_loadfactor:
+            limit = False
+            loadfactor = float(os.getloadavg()[0]) / os.cpu_count()
+            # bb.warn("Comparing %s to %s" % (loadfactor, self.rq.max_loadfactor))
+            if loadfactor > self.rq.max_loadfactor:
+                limit = True
+            if hasattr(self, "loadfactor_limit") and limit != self.loadfactor_limit:
+                bb.note("Load average limiting set to %s as load average: %s - using %s/%s bitbake threads" % (limit, loadfactor, len(self.rq.runq_running.difference(self.rq.runq_complete)), self.rq.number_tasks))
+            self.loadfactor_limit = limit
+            return limit
         return False
 
     def next_buildable_task(self):
@@ -1822,6 +1832,7 @@ class RunQueueExecute:
         self.max_cpu_pressure = self.cfgData.getVar("BB_PRESSURE_MAX_CPU")
         self.max_io_pressure = self.cfgData.getVar("BB_PRESSURE_MAX_IO")
         self.max_memory_pressure = self.cfgData.getVar("BB_PRESSURE_MAX_MEMORY")
+        self.max_loadfactor = self.cfgData.getVar("BB_LOADFACTOR_MAX")
 
         self.sq_buildable = set()
         self.sq_running = set()
@@ -1875,6 +1886,11 @@ class RunQueueExecute:
                 bb.fatal("Invalid BB_PRESSURE_MAX_MEMORY %s, minimum value is %s." % (self.max_memory_pressure, lower_limit))
             if self.max_memory_pressure > upper_limit:
                 bb.warn("Your build will be largely unregulated since BB_PRESSURE_MAX_MEMORY is set to %s. It is very unlikely that such high pressure will be experienced." % (self.max_io_pressure))
+
+        if self.max_loadfactor:
+            self.max_loadfactor = float(self.max_loadfactor)
+            if self.max_loadfactor <= 0:
+                bb.fatal("Invalid BB_LOADFACTOR_MAX %s, needs to be greater than zero." % (self.max_loadfactor))
             
         # List of setscene tasks which we've covered
         self.scenequeue_covered = set()