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

From: Dietmar Eggemann
Date: Wed Aug 29 2018 - 06:55:05 EST

On 08/28/2018 03:53 PM, Dietmar Eggemann wrote:
On 08/27/2018 12:14 PM, Peter Zijlstra wrote:
On Fri, Aug 24, 2018 at 02:24:48PM -0700, Steve Muckle wrote:
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

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.

See, I'm confused...

The only time TASK_WAKING is visible, is if we've done a remote wakeup
and it's 'stuck' on the remote wake_list. And in that case we've done
migrate_task_rq_fair() on it.

So by the time either rt_mutex_setprio() or __sched_setscheduler() get
to calling check_class_changed(), under both pi_lock and rq->lock, the
vruntime_normalized() thing should be right.

So please detail the exact scenario. Because I'm not seeing it.

Using Steve's test program ( I see the
issue but only if the two tasks (rt_task, fair_task) run on 2 cpus which
don't share LLC (e.g. CPU0 and CPU4 on hikey960).

So the wakeup goes the TTWU_QUEUE && !share_cache (ttwu_queue_remote) path.

I forgot to mention that since fair_task's cpu affinity is restricted to CPU4, there is no call to set_task_cpu()->migrate_task_rq_fair() since if (task_cpu(p) != cpu) fails.

I think the combination of cpu affinity of the fair_task to CPU4 and the fact that the scheduler runs on CPU1 when waking fair_task (with the two cpus not sharing LLC) while TTWU_QUEUE is enabled is the situation in which this vruntime issue can happen.