6.18-stable review patch. If anyone has any objections, please let me know.

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From: Peter Zijlstra peterz@infradead.org

[ Upstream commit d206fbad9328ddb68ebabd7cf7413392acd38081 ]

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 Signed-off-by: Sasha Levin sashal@kernel.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 71d3f4125b0e7..967ca52fb2311 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -12174,14 +12174,8 @@ 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)) { /* @@ -12873,17 +12867,10 @@ 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); }

/*