sched/eevdf: Also update slice on placement

Tasks that never consume their full slice would not update their slice value.
This means that tasks that are spawned before the sysctl scaling keep their
original (UP) slice length.

Fixes: 147f3efa ("sched/fair: Implement an EEVDF-like scheduling policy")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230915124822.847197830@noisy.programming.kicks-ass.net

kernel/sched/fair.c

@@ -4919,10 +4919,12 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq) {}
 static void
 place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 {
-	u64 vslice = calc_delta_fair(se->slice, se);
-	u64 vruntime = avg_vruntime(cfs_rq);
+	u64 vslice, vruntime = avg_vruntime(cfs_rq);
 	s64 lag = 0;
 
+	se->slice = sysctl_sched_base_slice;
+	vslice = calc_delta_fair(se->slice, se);
+
 	/*
 	 * Due to how V is constructed as the weighted average of entities,
 	 * adding tasks with positive lag, or removing tasks with negative lag