Re: [PATCH Resend v6] sched: fix wrong rq's runnable_avg update with rt tasks

[Mike Galbraith <efault@gmx.de>]

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
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