sched/fair: Prevent negative lag increase during delayed dequeue

sched/fair: Prevent negative lag increase during delayed dequeue

Delayed dequeue feature aims to reduce the negative lag of a dequeued
task while sleeping but it can happens that newly enqueued tasks will
move backward the avg vruntime and increase its negative lag.
When the delayed dequeued task wakes up, it has more neg lag compared
to being dequeued immediately or to other tasks that have been
dequeued just before theses new enqueues.

Ensure that the negative lag of a delayed dequeued task doesn't
increase during its delayed dequeued phase while waiting for its neg
lag to diseappear. Similarly, we remove any positive lag that the
delayed dequeued task could have gain during thsi period.

Short slice tasks are particularly impacted in overloaded system.

Test on snapdragon rb5:

hackbench -T -p -l 16000000 -g 2 1> /dev/null &
cyclictest -t 1 -i 2777 -D 333 --policy=fair --mlock  -h 20000 -q

The scheduling latency of cyclictest is:

                       tip/sched/core  tip/sched/core    +this patch
cyclictest slice  (ms) (default)2.8             8               8
hackbench slice   (ms) (default)2.8            20              20
Total Samples          |   115632          119733          119806
Average           (us) |      364              64(-82%)        61(- 5%)
Median (P50)      (us) |       60              56(- 7%)        56(  0%)
90th Percentile   (us) |     1166              62(-95%)        62(  0%)
99th Percentile   (us) |     4192              73(-98%)        72(- 1%)
99.9th Percentile (us) |     8528            2707(-68%)      1300(-52%)
Maximum           (us) |    17735           14273(-20%)     13525(- 5%)

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260331162352.551501-1-vincent.guittot@linaro.org