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

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Message-Id: <20240923072156.343623-1-yu.c.chen@intel.com>
Date: Mon, 23 Sep 2024 15:21:56 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Valentin Schneider <vschneid@...hat.com>,
	Chunxin Zang <zangchunxin@...iang.com>,
	linux-kernel@...r.kernel.org,
	Oliver Sang <oliver.sang@...el.com>,
	Chen Yu <yu.chen.surf@...il.com>,
	Chen Yu <yu.c.chen@...el.com>
Subject: [PATCH] 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. 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
Signed-off-by: Chen Yu <yu.c.chen@...el.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 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