Re: [PATCH] sched/fair: vruntime should normalize when switching from fair

[Steve Muckle <smuckle@google.com>]

On 08/24/2018 02:47 AM, Peter Zijlstra wrote:
>>> On 08/17/2018 11:27 AM, Steve Muckle wrote:
> 
>>>> When rt_mutex_setprio changes a task's scheduling class to RT,
>>>> we're seeing cases where the task's vruntime is not updated
>>>> correctly upon return to the fair class.
> 
>>>> Specifically, the following is being observed:
>>>> - task is deactivated while still in the fair class
>>>> - task is boosted to RT via rt_mutex_setprio, which changes
>>>>     the task to RT and calls check_class_changed.
>>>> - check_class_changed leads to detach_task_cfs_rq, at which point
>>>>     the vruntime_normalized check sees that the task's state is TASK_WAKING,
>>>>     which results in skipping the subtraction of the rq's min_vruntime
>>>>     from the task's vruntime
>>>> - later, when the prio is deboosted and the task is moved back
>>>>     to the fair class, the fair rq's min_vruntime is added to
>>>>     the task's vruntime, even though it wasn't subtracted earlier.
> 
> I'm thinking that is an incomplete scenario; where do we get to
> TASK_WAKING.

Yes there's a missing bit of context here at the beginning that the task 
to be boosted had already been put into TASK_WAKING.