nohz/full, sched/rt: Fix missed tick-reenabling bug in dequeue_task_rt()

Bugzilla: https://bugzilla.redhat.com/2107236

commit 5c66d1b9b30f737fcef85a0b75bfe0590e16b62a
Author: Nicolas Saenz Julienne nsaenzju@redhat.com
Date: Tue Jun 28 11:22:59 2022 +0200

nohz/full, sched/rt: Fix missed tick-reenabling bug in dequeue_task_rt()  

dequeue_task_rt() only decrements 'rt_rq->rt_nr_running' after having  
called sched_update_tick_dependency() preventing it from re-enabling the  
tick on systems that no longer have pending SCHED_RT tasks but have  
multiple runnable SCHED_OTHER tasks:  

  dequeue_task_rt()  
    dequeue_rt_entity()  
      dequeue_rt_stack()  
        dequeue_top_rt_rq()  
          sub_nr_running()      // decrements rq->nr_running  
            sched_update_tick_dependency()  
              sched_can_stop_tick()     // checks rq->rt.rt_nr_running,  
              ...  
        __dequeue_rt_entity()  
          dec_rt_tasks()        // decrements rq->rt.rt_nr_running  
          ...  

Every other scheduler class performs the operation in the opposite  
order, and sched_update_tick_dependency() expects the values to be  
updated as such. So avoid the misbehaviour by inverting the order in  
which the above operations are performed in the RT scheduler.  

Fixes: 76d92ac305f2 ("sched: Migrate sched to use new tick dependency mask model")  
Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>  
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>  
Reviewed-by: Valentin Schneider <vschneid@redhat.com>  
Reviewed-by: Phil Auld <pauld@redhat.com>  
Link: https://lore.kernel.org/r/20220628092259.330171-1-nsaenzju@redhat.com  

Signed-off-by: Phil Auld pauld@redhat.com