On 19 April 2013 10:14, Mike Galbraith <efa...@gmx.de> wrote: > 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
I'm not sure that such behavior that is only based on erroneous value, is good one. > 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. The target is not perfect number but good enough to be usable. The systctl_migration_cost threshold is good for idle balancing but can generates wrong load 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/
