sched/fair: Manage lag and run to parity with different slices

This follows the attempt to better track maximum lag of task in presence
of different slices duration:
[1]  https://lore.kernel.org/all/20250418151225.3006867-1-vinc...

Patch 1 is a simple cleanup to ease following changes.

Patch 2 uses Peter's proposal made in [1] to track the max slice of
enqueued tasks and use it to clamp the lag of dequeued task.

Patch 3 modify the protection of the slice of the current task to take
into account case when tasks with shorter slice wait to run on the cpu

Patch 4 extend to slice protection mecanism to the NO_RUN_TO_PARITY case
to ensure that a resched will be set regularly. Currently, the resched 
will be set only when curr will elapse its slice which is partly similar
to RUN_TO_PARITY. Now the running task has a minimum time quantum (0.7ms)
before eevdf looks for another task to run with the exception of
PREEMPT_SHORT case which remains valid.

Peter Zijlstra (1):
  sched/fair: Increase max lag clamping

Vincent Guittot (3):
  sched/fair: Use protect_slice() instead of direct comparison
  sched/fair: Limit run to parity to the min slice of enqueued entities
  sched/fair: Improve NO_RUN_TO_PARITY

 include/linux/sched.h |  1 +
 kernel/sched/fair.c   | 76 ++++++++++++++++++++++++++++++++++---------
 2 files changed, 61 insertions(+), 16 deletions(-)

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2.43.0