[PATCH 2/5] sched/fair: Avoid rq->lock bouncing in sched_balance_newidle()

Peter Zijlstra
2025-11-27T15:48:14Z

While poking at this code recently I noted we do a pointless
unlock+lock cycle in sched_balance_newidle(). We drop the rq->lock (so
we can balance) but then instantly grab the same rq->lock again in
sched_balance_update_blocked_averages().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/sched/fair.c | 27 ++++++++++++++++++---------
1 file changed, 18 insertions(+), 9 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9902,15 +9902,11 @@ static unsigned long task_h_load(struct
}
#endif /* !CONFIG_FAIR_GROUP_SCHED */

-static void sched_balance_update_blocked_averages(int cpu)
+static void __sched_balance_update_blocked_averages(struct rq *rq)
{
bool decayed = false, done = true;
- struct rq *rq = cpu_rq(cpu);
- struct rq_flags rf;

- rq_lock_irqsave(rq, &rf);
update_blocked_load_tick(rq);
- update_rq_clock(rq);

decayed |= __update_blocked_others(rq, &done);
decayed |= __update_blocked_fair(rq, &done);
@@ -9918,7 +9914,15 @@ static void sched_balance_update_blocked
update_blocked_load_status(rq, !done);
if (decayed)
cpufreq_update_util(rq, 0);
- rq_unlock_irqrestore(rq, &rf);
+}
+
+static void sched_balance_update_blocked_averages(int cpu)
+{
+ struct rq *rq = cpu_rq(cpu);
+
+ guard(rq_lock_irqsave)(rq);
+ update_rq_clock(rq);
+ __sched_balance_update_blocked_averages(rq);
}

/********** Helpers for sched_balance_find_src_group ************************/
@@ -12865,12 +12869,17 @@ static int sched_balance_newidle(struct
}
rcu_read_unlock();

+ /*
+ * Include sched_balance_update_blocked_averages() in the cost
+ * calculation because it can be quite costly -- this ensures we skip
+ * it when avg_idle gets to be very low.
+ */
+ t0 = sched_clock_cpu(this_cpu);
+ __sched_balance_update_blocked_averages(this_rq);
+
rq_modified_clear(this_rq);
raw_spin_rq_unlock(this_rq);

- t0 = sched_clock_cpu(this_cpu);
- sched_balance_update_blocked_averages(this_cpu);
-
rcu_read_lock();
for_each_domain(this_cpu, sd) {
u64 domain_cost;