linux-kernel - [PATCH] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle

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Date:   Tue, 23 Nov 2021 19:22:29 +0800
From:   Barry Song <21cnbao@...il.com>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org
Cc:     dietmar.eggemann@....com, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, Barry Song <song.bao.hua@...ilicon.com>
Subject: [PATCH] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu

From: Barry Song <song.bao.hua@...ilicon.com>

This patch keeps the same scanning amount, but drops a redundant loop
of cpumask_next_wrap.
The original code did for_each_cpu_wrap(cpu, cpus, target + 1), then
checked --nr; this patch does --nr before doing the next loop, thus,
it can remove a cpumask_next_wrap() which costs a little bit.

Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
---
 kernel/sched/fair.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ff69f24..e2fb3e0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6298,9 +6298,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 
 		span_avg = sd->span_weight * avg_idle;
 		if (span_avg > 4*avg_cost)
-			nr = div_u64(span_avg, avg_cost);
+			nr = div_u64(span_avg, avg_cost) - 1;
 		else
-			nr = 4;
+			nr = 3;
 
 		time = cpu_clock(this);
 	}
@@ -6312,11 +6312,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 				return i;
 
 		} else {
-			if (!--nr)
-				return -1;
 			idle_cpu = __select_idle_cpu(cpu, p);
 			if ((unsigned int)idle_cpu < nr_cpumask_bits)
 				break;
+			if (!--nr)
+				return -1;
 		}
 	}
 
-- 
1.8.3.1