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

[Peter Zijlstra <peterz@infradead.org>]

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.
> 
> A new idle_exit function is called when the prev task is the idle function
> so the elapsed time will be accounted as idle time in the rq's load.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

Thanks Vince!