On Fri, 2013-04-19 at 09:49 +0200, Vincent Guittot wrote: > 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.
If it only scares fair class tasks away from the periodic rt load, that seems like a benefit to me, not a liability. If we really really need perfect load numbers, fine, we have to eat some cycles, but when I look at it, it looks like one of those "Perfect is the enemy of good" things. -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/