sched/eevdf: Fix wakeup-preempt by checking cfs_rq->nr_running

Commit 85e511d ("sched/eevdf: Allow shorter slices to wakeup-preempt")
introduced a mechanism that a wakee with shorter slice could preempt
the current running task. It also lower the bar for the current task
to be preempted, by checking the rq->nr_running instead of cfs_rq->nr_running
when the current task has ran out of time slice. But there is a scenario
that is problematic. Say, if there is 1 cfs task and 1 rt task, before
85e511d, update_deadline() will not trigger a reschedule, and after
85e511d, since rq->nr_running is 2 and resched is true, a resched_curr()
would happen.

Some workloads (like the hackbench reported by lkp) do not like
over-scheduling. We can see that the preemption rate has been
increased by 2.2%:

1.654e+08            +2.2%   1.69e+08        hackbench.time.involuntary_context_switches

Restore its previous check criterion.

Fixes: 85e511d ("sched/eevdf: Allow shorter slices to wakeup-preempt")
Closes: https://lore.kernel.org/oe-lkp/[email protected]
Reported-by: kernel test robot <[email protected]>
Suggested-by: K Prateek Nayak <[email protected]>
Signed-off-by: Chen Yu <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Honglei Wang <[email protected]>
Tested-by: K Prateek Nayak <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: yu-chen-surf

kernel/sched/fair.c

@@ -1247,7 +1247,7 @@ static void update_curr(struct cfs_rq *cfs_rq)
 
 	account_cfs_rq_runtime(cfs_rq, delta_exec);
 
-	if (rq->nr_running == 1)
+	if (cfs_rq->nr_running == 1)
 		return;
 
 	if (resched || did_preempt_short(cfs_rq, curr)) {