[tip: sched/core] sched/fair: Revert max_newidle_lb_cost bump

tip-bot2 for Peter Zijlstra
2025-11-14T12:19:05Z

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@infradead.org>
AuthorDate: Fri, 07 Nov 2025 17:01:20 +01:00
Committer: Peter Zijlstra <peterz@infradead.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@oracle.com>
Reported-by: Adam Li <adamli@os.amperecomputing.com>
Reported-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reported-by: Hazem Mohamed Abuelfotoh <abuehaze@amazon.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Chris Mason <clm@meta.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);
}

/*