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

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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 71fedc41c2 ("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
This commit is contained in:
K Prateek Nayak
2026-03-12 04:44:32 +00:00
committed by Peter Zijlstra
parent 8ca12326f5
commit fa6874dfee
1 changed files with 12 additions and 21 deletions
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33
kernel/sched/fair.c
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@@ -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;
}