On Thu, 2013-04-18 at 18:34 +0200, Vincent Guittot wrote: > The current update of the rq's load can be erroneous when RT tasks are > involved > > The update of the load of a rq that becomes idle, is done only if the avg_idle > is less than sysctl_sched_migration_cost. If RT tasks and short idle duration > alternate, the runnable_avg will not be updated correctly and the time will be > accounted as idle time when a CFS task wakes up. > > A new idle_enter function is called when the next task is the idle function > so the elapsed time will be accounted as run time in the load of the rq, > whatever the average idle time is. The function update_rq_runnable_avg is > removed from idle_balance. > > When a RT task is scheduled on an idle CPU, the update of the rq's load is > not done when the rq exit idle state because CFS's functions are not > called. Then, the idle_balance, which is called just before entering the > idle function, updates the rq's load and makes the assumption that the > elapsed time since the last update, was only running time. > > As a consequence, the rq's load of a CPU that only runs a periodic RT task, > is close to LOAD_AVG_MAX whatever the running duration of the RT task is. > > A new idle_exit function is called when the prev task is the idle function > so the elapsed time will be accounted as idle time in the rq's load.
Acked-by: Peter Zijlstra <a.p.zijls...@chello.nl> Thanks Vince! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
