On 19 April 2013 06:30, Mike Galbraith <efa...@gmx.de> wrote: > 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. > > Why do we care what rq's load says, if the only thing running is a > periodic RT task? I _think_ I recall that stuff being put under the
cfs scheduler will use a wrong rq load the next time it wants to schedule a task > throttle specifically to not waste cycles doing that on every > microscopic idle. yes but this lead to the wrong computation of runnable_avg_sum. To be more precise, we only need to call __update_entity_runnable_avg, __update_tg_runnable_avg is not mandatory in this step. > > Seems to me when scheduling an rt task, you want to do as little other > than switching to/from the rt task as possible. I don't let rt tasks do > idle balancing either, their job isn't to balance fair class on the way > out the door, it's to get off/onto the cpu ASAP, and do rt work. I agree but the patch is not about balancing fair task but keep coherent runnable value Vincent > > -Mike > -- 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/