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

["tip-bot2 for Chen Yu" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     d4ac164bde7a12ec0a238a7ead5aa26819bbb1c1
Gitweb:        https://git.kernel.org/tip/d4ac164bde7a12ec0a238a7ead5aa26819bbb1c1
Author:        Chen Yu <yu.c.chen@intel.com>
AuthorDate:    Wed, 25 Sep 2024 16:54:40 +08:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 02 Oct 2024 11:27:54 +02:00

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

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. But there is a scenario
that is problematic. 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")
Closes: https://lore.kernel.org/oe-lkp/202409231416.9403c2e9-oliver.sang@intel.com
Reported-by: kernel test robot <oliver.sang@intel.com>
Suggested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Honglei Wang <jameshongleiwang@126.com>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://lore.kernel.org/r/20240925085440.358138-1-yu.c.chen@intel.com
---
 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 b63a7ac..ab497fa 100644
--- a/kernel/sched/fair.c
+++ b/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)) {