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

[Chen Yu]

Commit 85e511df3cec ("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. Say, if there is 1 cfs
task and 1 rt task, before 85e511df3cec, update_deadline() will
not trigger a reschedule, and after 85e511df3cec, 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: 85e511df3cec ("sched/eevdf: Allow shorter slices to wakeup-preempt")
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202409231416.9403c2e9-oliver.sang@xxxxxxxxx
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 225b31aaee55..2859fc7e2da2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1025,7 +1025,7 @@ static bool update_deadline(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	/*
 	 * The task has consumed its request, reschedule.
 	 */
-	return true;
+	return (cfs_rq->nr_running > 1);
 }
 
 #include "pelt.h"
-- 
2.25.1