[PATCH 3/5] sched: Change rcu_dereference_check_sched_domain() to rcu-sched

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

By changing rcu_dereference_check_sched_domain() to use
rcu_dereference_sched_check() it also considers preempt_disable() to
be equivalent to rcu_read_lock().

Since rcu fully implies rcu_sched this has absolutely no change in
behaviour, but it does allow removing a bunch of otherwise redundant
rcu_read_lock() noise.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/sched/fair.c | 9 +--------
kernel/sched/sched.h | 2 +-
2 files changed, 2 insertions(+), 9 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12853,21 +12853,16 @@ static int sched_balance_newidle(struct
*/
rq_unpin_lock(this_rq, rf);

- rcu_read_lock();
sd = rcu_dereference_check_sched_domain(this_rq->sd);
- if (!sd) {
- rcu_read_unlock();
+ if (!sd)
goto out;
- }

if (!get_rd_overloaded(this_rq->rd) ||
this_rq->avg_idle < sd->max_newidle_lb_cost) {

update_next_balance(sd, &next_balance);
- rcu_read_unlock();
goto out;
}
- rcu_read_unlock();

/*
* Include sched_balance_update_blocked_averages() in the cost
@@ -12880,7 +12875,6 @@ static int sched_balance_newidle(struct
rq_modified_clear(this_rq);
raw_spin_rq_unlock(this_rq);

- rcu_read_lock();
for_each_domain(this_cpu, sd) {
u64 domain_cost;

@@ -12930,7 +12924,6 @@ static int sched_balance_newidle(struct
if (pulled_task || !continue_balancing)
break;
}
- rcu_read_unlock();

raw_spin_rq_lock(this_rq);

--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2009,7 +2009,7 @@ queue_balance_callback(struct rq *rq,
}

#define rcu_dereference_check_sched_domain(p) \
- rcu_dereference_check((p), lockdep_is_held(&sched_domains_mutex))
+ rcu_dereference_sched_check((p), lockdep_is_held(&sched_domains_mutex))

/*
* The domain tree (rq->sd) is protected by RCU's quiescent state transition.