linux-kernel - [PATCH v3] sched/fair: Eliminate confusion for the scan number in select_idle

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Date:   Fri, 26 Nov 2021 18:51:39 +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 v3] sched/fair: Eliminate confusion for the scan number in select_idle_cpu

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

Right now, the number of scans is very confusing. For example, if nr==4,
the code is iterating 4 times but scanning 3 times only. Because in the
4th iteration, the code returns directly due to --nr==0. So the code is
wasting time on doing the 4th cpumask_next_wrap() and it is also very
confusing since the actual scan number is nr-1 but not nr. This seems not
to be the original intention.

This patch moves to scan nr times rather than nr-1 times, and it also
makes the last iteration of cpumask_next_wrap() useful.

Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
---
 -v3:
  scan nr times instead of nr-1 times with respect to Peter's comments;
  rewrite commit log and subject since we have changed the behaviour of
  the code;
 -v2:
  https://lore.kernel.org/lkml/20211124091546.5072-1-21cnbao@gmail.com/

 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6e476f6..5c259aa 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6285,11 +6285,11 @@ 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