linux-kernel - [PATCH 1/4] sched/fair: Remove SIS_AVG

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Message-Id: <20201207091516.24683-2-mgorman@techsingularity.net>
Date:   Mon,  7 Dec 2020 09:15:13 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Ziljstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux-ARM <linux-arm-kernel@...ts.infradead.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU

SIS_AVG_CPU was introduced as a means of avoiding a search when the
average search cost indicated that the search would likely fail. It
was a blunt instrument and disabled by 4c77b18cf8b7 ("sched/fair: Make
select_idle_cpu() more aggressive") and later replaced with a proportional
search depth by 1ad3aaf3fcd2 ("sched/core: Implement new approach to
scale select_idle_cpu()").

While there are corner cases where SIS_AVG_CPU is better, it has now been
disabled for almost three years. As the intent of SIS_PROP is to reduce
the time complexity of select_idle_cpu(), lets drop SIS_AVG_CPU and focus
on SIS_PROP as a throttling mechanism.

Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
---
 kernel/sched/fair.c     | 3 ---
 kernel/sched/features.h | 1 -
 2 files changed, 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 98075f9ea9a8..23934dbac635 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6161,9 +6161,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 	avg_idle = this_rq()->avg_idle / 512;
 	avg_cost = this_sd->avg_scan_cost + 1;
 
-	if (sched_feat(SIS_AVG_CPU) && avg_idle < avg_cost)
-		return -1;
-
 	if (sched_feat(SIS_PROP)) {
 		u64 span_avg = sd->span_weight * avg_idle;
 		if (span_avg > 4*avg_cost)
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 68d369cba9e4..e875eabb6600 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -54,7 +54,6 @@ SCHED_FEAT(TTWU_QUEUE, true)
 /*
  * When doing wakeups, attempt to limit superfluous scans of the LLC domain.
  */
-SCHED_FEAT(SIS_AVG_CPU, false)
 SCHED_FEAT(SIS_PROP, true)
 
 /*
-- 
2.26.2