sched/fair: Remove superfluous rcu_read_lock() in the wakeup path

select_task_rq_fair() is always called with p->pi_lock held and IRQs
disabled which makes it equivalent of an RCU read-side.

Since commit 71fedc41c23b ("sched/fair: Switch to
rcu_dereference_all()") switched to using rcu_dereference_all() in the
wakeup path, drop the explicit rcu_read_{lock,unlock}() in the fair
task's wakeup path.

Future plans to reuse select_task_rq_fair() /
find_energy_efficient_cpu() in the fair class' balance callback will do
so with IRQs disabled and will comply with the requirements of
rcu_dereference_all() which makes this safe keeping in mind future
development plans too.

Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.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>
Link: https://patch.msgid.link/20260312044434.1974-8-kprateek.nayak@amd.com

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c1e5c8298fd17effd9bb7eb9a79caea67bddd0b7..3e24d3e16522c38bdcd5bab692083cfb774f8530 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8570,10 +8570,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
        struct perf_domain *pd;
        struct energy_env eenv;
 
-       rcu_read_lock();
        pd = rcu_dereference_all(rd->pd);
        if (!pd)
-               goto unlock;
+               return target;
 
        /*
         * Energy-aware wake-up happens on the lowest sched_domain starting
@@ -8583,13 +8582,13 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
        while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd)))
                sd = sd->parent;
        if (!sd)
-               goto unlock;
+               return target;
 
        target = prev_cpu;
 
        sync_entity_load_avg(&p->se);
        if (!task_util_est(p) && p_util_min == 0)
-               goto unlock;
+               return target;
 
        eenv_task_busy_time(&eenv, p, prev_cpu);
 
@@ -8684,7 +8683,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
                                                    prev_cpu);
                        /* CPU utilization has changed */
                        if (prev_delta < base_energy)
-                               goto unlock;
+                               return target;
                        prev_delta -= base_energy;
                        prev_actual_cap = cpu_actual_cap;
                        best_delta = min(best_delta, prev_delta);
@@ -8708,7 +8707,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
                                                   max_spare_cap_cpu);
                        /* CPU utilization has changed */
                        if (cur_delta < base_energy)
-                               goto unlock;
+                               return target;
                        cur_delta -= base_energy;
 
                        /*
@@ -8725,7 +8724,6 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
                        best_actual_cap = cpu_actual_cap;
                }
        }
-       rcu_read_unlock();
 
        if ((best_fits > prev_fits) ||
            ((best_fits > 0) && (best_delta < prev_delta)) ||
@@ -8733,11 +8731,6 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
                target = best_energy_cpu;
 
        return target;
-
-unlock:
-       rcu_read_unlock();
-
-       return target;
 }
 
 /*
@@ -8782,7 +8775,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
                want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, p->cpus_ptr);
        }
 
-       rcu_read_lock();
        for_each_domain(cpu, tmp) {
                /*
                 * If both 'cpu' and 'prev_cpu' are part of this domain,
@@ -8808,14 +8800,13 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
                        break;
        }
 
-       if (unlikely(sd)) {
-               /* Slow path */
-               new_cpu = sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag);
-       } else if (wake_flags & WF_TTWU) { /* XXX always ? */
-               /* Fast path */
-               new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
-       }
-       rcu_read_unlock();
+       /* Slow path */
+       if (unlikely(sd))
+               return sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag);
+
+       /* Fast path */
+       if (wake_flags & WF_TTWU)
+               return select_idle_sibling(p, prev_cpu, new_cpu);
 
        return new_cpu;
 }