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

[Steven Rostedt]

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.
> 
> Changes since V5:
> - Rename idle_enter/exit function to idle_enter/exit_fair
> 
> Changes since V4:
> - Rebase on v3.9-rc6 instead of Steven Rostedt's patches

Acked-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

-- Steve

> - Create the post_schedule_idle function that was previously created by Steven's patches
> 
> Changes since V3:
> - Remove dependancy with CONFIG_FAIR_GROUP_SCHED
> - Add a new idle_enter function and create a post_schedule callback for
>  idle class
> - Remove the update_runnable_avg from idle_balance
> 
> Changes since V2:
> - remove useless definition for UP platform
> - rebased on top of Steven Rostedt's patches :
> https://lkml.org/lkml/2013/2/12/558
> 
> Changes since V1:
> - move code out of schedule function and create a pre_schedule callback for
>   idle class instead.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---


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