[PATCH v2 0/4] sched/fair: Active balancer RT/DL preemption fix

[Valentin Schneider <valentin.schneider@arm.com>]

Vincent's load balance rework [1] got me thinking about how and where we
use rq.nr_running vs rq.cfs.h_nr_running checks, and this lead me to
stare intently at the active load balancer.

I haven't seen it happen (yet), but from reading the code it really looks
like we can have some scenarios where the cpu_stopper ends up preempting
a > CFS class task, since we never actually look at what's the remote rq's
running task.

This series shuffles things around the CFS active load balancer to prevent
this from happening.

- Patch 1 is a freebie cleanup
- Patch 2 is a preparatory code move
- Patch 3 adds h_nr_running checks
- Patch 4 adds a sched class check + detach_one_task() to the active balance

This is based on top of today's tip/sched/core:
  a46d14eca7b7 ("sched/fair: Use rq_lock/unlock in online_fair_sched_group")

v1 -> v2:
  - (new patch) Added need_active_balance() cleanup

  - Tweaked active balance code move to respect existing
    sd->nr_balance_failed modifications
  - Added explicit checks of active_load_balance()'s return value
  
  - Added an h_nr_running < 1 check before kicking the cpu_stopper

  - Added a detach_one_task() call in active_load_balance() when the remote
    rq's running task is > CFS

[1]: https://lore.kernel.org/lkml/1564670424-26023-1-git-send-email-vincent.guittot@linaro.org/

Valentin Schneider (4):
  sched/fair: Make need_active_balance() return bools
  sched/fair: Move active balance logic to its own function
  sched/fair: Check for CFS tasks before detach_one_task()
  sched/fair: Prevent active LB from preempting higher sched classes

 kernel/sched/fair.c | 151 ++++++++++++++++++++++++++++----------------
 1 file changed, 95 insertions(+), 56 deletions(-)

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2.22.0