linux-kernel - [tip: sched/core] sched/fair: Revert max_newidle_lb

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Message-ID: <176312274291.498.4509981768126901496.tip-bot2@tip-bot2>
Date: Fri, 14 Nov 2025 12:19:02 -0000
From: "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Joseph Salisbury <joseph.salisbury@...cle.com>,
 Adam Li <adamli@...amperecomputing.com>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, Chris Mason <clm@...a.com>,
 x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched/fair: Revert max_newidle_lb_cost bump

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     63bdc220840d16b370fdf678b7d916552fef046e
Gitweb:        https://git.kernel.org/tip/63bdc220840d16b370fdf678b7d916552fef046e
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Fri, 07 Nov 2025 17:01:20 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 14 Nov 2025 13:03:07 +01:00

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>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@....com>
Tested-by: Chris Mason <clm@...a.com>
Link: https://lkml.kernel.org/r/20250626144017.1510594-2-clm@fb.com
Link: https://lkml.kernel.org/r/006c9df2-b691-47f1-82e6-e233c3f91faf@oracle.com
Link: https://patch.msgid.link/20251107161739.406147760@infradead.org
---
 kernel/sched/fair.c | 19 +++----------------
 1 file changed, 3 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 11d480e..bfb8935 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12229,14 +12229,8 @@ static inline bool update_newidle_cost(struct sched_domain *sd, u64 cost)
 		/*
 		 * 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)) {
 		/*
@@ -12919,17 +12913,10 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
 
 			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);
 		}
 
 		/*