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

[Miguel de Dios]

On 08/17/2018 11:27 AM, Steve Muckle wrote:
> From: John Dias <joaodias@xxxxxxxxxx>
>
> 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.
> The immediate result is inflation of the task's vruntime, giving
> it lower priority (starving it if there's enough available work).
> The longer-term effect is inflation of all vruntimes because the
> task's vruntime becomes the rq's min_vruntime when the higher
> priority tasks go idle. That leads to a vicious cycle, where
> the vruntime inflation repeatedly doubled.
>
> The change here is to detect when vruntime_normalized is being
> called when the task is waking but is waking in another class,
> and to conclude that this is a case where vruntime has not
> been normalized.
>
> Signed-off-by: John Dias <joaodias@xxxxxxxxxx>
> Signed-off-by: Steve Muckle <smuckle@xxxxxxxxxx>
> ---
>   kernel/sched/fair.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b39fb596f6c1..14011d7929d8 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9638,7 +9638,8 @@ static inline bool vruntime_normalized(struct task_struct *p)
>   	 * - A task which has been woken up by try_to_wake_up() and
>   	 *   waiting for actually being woken up by sched_ttwu_pending().
>   	 */
> -	if (!se->sum_exec_runtime || p->state == TASK_WAKING)
> +	if (!se->sum_exec_runtime ||
> +	    (p->state == TASK_WAKING && p->sched_class == &fair_sched_class))
>   		return true;
>   
>   	return false;
The normalization of vruntime used to exist in task_waking but it was 
removed and the normalization was moved into migrate_task_rq_fair. The 
reasoning being that task_waking_fair was only hit when a task is queued 
onto a different core and migrate_task_rq_fair should do the same work.

However, we're finding that there's one case which migrate_task_rq_fair 
doesn't hit: that being the case where rt_mutex_setprio changes a task's 
scheduling class to RT when its scheduled out. The task never hits 
migrate_task_rq_fair because it is switched to RT and migrates as an RT 
task. Because of this we're getting an unbounded addition of 
min_vruntime when the task is re-attached to the CFS runqueue when it 
loses the inherited priority. The patch above works because now the 
kernel specifically checks for this case and normalizes accordingly.

Here's the patch I was talking about: 
https://lore.kernel.org/patchwork/patch/677689/. In our testing we were 
seeing vruntimes nearly double every time after rt_mutex_setprio boosts 
the task to RT.

Signed-off-by: Miguel de Dios <migueldedios@xxxxxxxxxx>
Tested-by: Miguel de Dios <migueldedios@xxxxxxxxxx>