Messages in this thread | | | Subject | Re: [PATCH Resend v6] sched: fix wrong rq's runnable_avg update with rt tasks | From | Mike Galbraith <> | Date | Fri, 19 Apr 2013 11:21:44 +0200 |
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On Fri, 2013-04-19 at 10:50 +0200, Vincent Guittot wrote: > On 19 April 2013 10:14, Mike Galbraith <efault@gmx.de> wrote: > > On Fri, 2013-04-19 at 09:49 +0200, Vincent Guittot wrote: > >> On 19 April 2013 06:30, Mike Galbraith <efault@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. 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 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
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