linux-kernel - [PATCH v2] sched/eevdf: Fix wakeup-preempt by checking cfs_rq->nr

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Message-Id: <20240925085440.358138-1-yu.c.chen@intel.com>
Date: Wed, 25 Sep 2024 16:54:40 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
	Valentin Schneider <vschneid@...hat.com>,
	Chunxin Zang <zangchunxin@...iang.com>,
	linux-kernel@...r.kernel.org,
	Chen Yu <yu.chen.surf@...il.com>,
	Chen Yu <yu.c.chen@...el.com>,
	kernel test robot <oliver.sang@...el.com>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: [PATCH v2] 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")
Reported-by: kernel test robot <oliver.sang@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202409231416.9403c2e9-oliver.sang@intel.com
Suggested-by: K Prateek Nayak <kprateek.nayak@....com>
Tested-by: K Prateek Nayak <kprateek.nayak@....com>
Signed-off-by: Chen Yu <yu.c.chen@...el.com>
---
v1->v2:
    Check cfs_rq->nr_running instead of rq->nr_running(K Prateek Nayak)
---
 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..53a351b18740 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)) {
-- 
2.25.1