linux-kernel - [PATCH 1/4] sched/fair: Revert max_newidle_lb

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Message-ID: <20251107161739.406147760@infradead.org>
Date: Fri, 07 Nov 2025 17:06:46 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Chris Mason <clm@...a.com>,
 Joseph Salisbury <joseph.salisbury@...cle.com>,
 Adam Li <adamli@...amperecomputing.com>,
 Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>,
 Josh Don <joshdon@...gle.com>
Cc: mingo@...hat.com,
 peterz@...radead.org,
 juri.lelli@...hat.com,
 vincent.guittot@...aro.org,
 dietmar.eggemann@....com,
 rostedt@...dmis.org,
 bsegall@...gle.com,
 mgorman@...e.de,
 vschneid@...hat.com,
 linux-kernel@...r.kernel.org
Subject: [PATCH 1/4] sched/fair: Revert max_newidle_lb_cost bump

Many people reported regressions on their database workloads due to:

  155213a2aed4 ("sched/fair: Bump sd->max_newidle_lb_cost when newidle balance fails")

For instance Adam Li reported a 6% regression on SpecJBB.

Conversely this will regress schbench again; on my machine from 2.22
Mrps/s down to 2.04 Mrps/s.

Reported-by: Joseph Salisbury <joseph.salisbury@...cle.com>
Reported-by: Adam Li <adamli@...amperecomputing.com>
Reported-by: Dietmar Eggemann <dietmar.eggemann@....com>
Reported-by: Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/sched/fair.c |   19 +++----------------
 1 file changed, 3 insertions(+), 16 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12152,14 +12152,8 @@ static inline bool update_newidle_cost(s
 		/*
 		 * Track max cost of a domain to make sure to not delay the
 		 * next wakeup on the CPU.
-		 *
-		 * sched_balance_newidle() bumps the cost whenever newidle
-		 * balance fails, and we don't want things to grow out of
-		 * control.  Use the sysctl_sched_migration_cost as the upper
-		 * limit, plus a litle extra to avoid off by ones.
 		 */
-		sd->max_newidle_lb_cost =
-			min(cost, sysctl_sched_migration_cost + 200);
+		sd->max_newidle_lb_cost = cost;
 		sd->last_decay_max_lb_cost = jiffies;
 	} else if (time_after(jiffies, sd->last_decay_max_lb_cost + HZ)) {
 		/*
@@ -12851,17 +12845,10 @@ static int sched_balance_newidle(struct
 
 			t1 = sched_clock_cpu(this_cpu);
 			domain_cost = t1 - t0;
+			update_newidle_cost(sd, domain_cost);
+
 			curr_cost += domain_cost;
 			t0 = t1;
-
-			/*
-			 * Failing newidle means it is not effective;
-			 * bump the cost so we end up doing less of it.
-			 */
-			if (!pulled_task)
-				domain_cost = (3 * sd->max_newidle_lb_cost) / 2;
-
-			update_newidle_cost(sd, domain_cost);
 		}
 
 		/*