On 19 April 2013 11:21, Mike Galbraith <efa...@gmx.de> wrote: > On Fri, 2013-04-19 at 10:50 +0200, Vincent Guittot wrote: >> 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 > > But again, why do we care? To be able to mix rt and fair loads and > still make pretty mixed load utilization numbers? Paying a general case
If runnable_avg_sum can be wrong, it becomes unusable and all the stuff around becomes useless. > fast path price to make strange (to me) load utilization numbers pretty > is not very attractive. If you muck about with rt classes, you need to > have a good reason for doing that. If you do have a good reason, you > also allocated all resources, including CPU, so don't need the kernel to Some tasks have responsiveness constraints so they use rt class but they also live with cfs tasks. Vincent > balance the load for you. Paying any fast path price to make the kernel > balance a mixed rt/fair load just seems fundamentally wrong to me. > > -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/
