On Fri, 2013-04-19 at 11:21 +0200, Mike Galbraith 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 > fast path price to make strange (to me) load utilization numbers pretty > is not very attractive.
So I'm not convinced this is a good thing to do, but it's not my call, that's Peter and Ingos job, so having expressed my opinion, I'll shut up and let them do their thing ;-) -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/