Re: [PATCH v4 7/9] sched/fair: Remove superfluous rcu_read_lock() in the wakeup path

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 12.03.26 05:44, K Prateek Nayak wrote:
> 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>
> ---
> Changelog v3..v4:
> 
> o No changes.
> ---
>  kernel/sched/fair.c | 33 ++++++++++++---------------------
>  1 file changed, 12 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d57c02e82f3a..28853c0abb83 100644
> --- 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);
Got an RCU related warning when running EAS:

[    3.795872] EM: rcu read lock needed
[    3.795903] WARNING: ./include/linux/energy_model.h:251 at compute_energy+0x3
a4/0x3bc, CPU#1: swapper/1/0
[    3.813755] Modules linked in:
[    3.816844] CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted 7.0.0-rc2-00295-
g93189edc73c8-dirty #30 PREEMPT
[    3.826807] Hardware name: ARM Juno development board (r0) (DT)
[    3.832752] pstate: 600000c5 (nZCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[    3.839750] pc : compute_energy+0x3a4/0x3bc
[    3.843970] lr : compute_energy+0x3a4/0x3bc
[    3.848185] sp : ffff8000838f39e0
[    3.851518] x29: ffff8000838f3a00 x28: ffff0008008fe560 x27: 00000000000000f9
[    3.858717] x26: ffff800082702250 x25: ffff000803211e00 x24: ffff8000838f3af8
[    3.865913] x23: ffff0008009a3580 x22: 00000000000000f9 x21: 00000000ffffffff
[    3.873108] x20: ffff0008030377c0 x19: 00000000000000b7 x18: 0000000000000028
[    3.880303] x17: ffff8008fc91f000 x16: ffff8000838f0000 x15: 0000000000000007
[    3.887497] x14: fffffffffffc73af x13: 0a64656465656e20 x12: 0000000000008000
[    3.894690] x11: ffff80008272ca18 x10: ffff8000825f7000 x9 : 0000000000000050
[    3.901884] x8 : 0000000000000008 x7 : ffff800080190100 x6 : 0000000000000001
[    3.909076] x5 : 0000000000000161 x4 : 0000000000000000 x3 : 0000000000000001
[    3.916268] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff0008002e1ac0
[    3.923461] Call trace:
[    3.925924]  compute_energy+0x3a4/0x3bc (P)
[    3.930147]  select_task_rq_fair+0x590/0x1990
[    3.934541]  try_to_wake_up+0x1f8/0xac4
[    3.938420]  wake_up_process+0x18/0x24
[    3.942208]  process_timeout+0x14/0x20
[    3.945995]  call_timer_fn+0xb8/0x470

We need to adapt em_cpu_energy() in include/linux/energy_model.h as
well.

---8<---

From 74f9067751b02f4bd0934ba6d47f2a204c763abe Mon Sep 17 00:00:00 2001
From: Dietmar Eggemann <dietmar.eggemann@arm.com>
Date: Sun, 15 Mar 2026 23:45:39 +0100
Subject: [PATCH] PM: EM: Switch to rcu_dereference_all() in wakeup path

em_cpu_energy() is part of the EAS (Fair) task wakeup path. Now that
rcu_read_{,un}lock() have been removed from find_energy_efficient_cpu()
switch to rcu_dereference_all() and check for rcu_read_lock_any_held()
in em_cpu_energy() as well.
In EAS (Fair) task wakeup path is a preempt/IRQ disabled region, so
rcu_read_{,un}lock() can be removed.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
 include/linux/energy_model.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
index e7497f804644..c909a8ba22e8 100644
--- a/include/linux/energy_model.h
+++ b/include/linux/energy_model.h
@@ -248,7 +248,7 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
 	struct em_perf_state *ps;
 	int i;
 
-	WARN_ONCE(!rcu_read_lock_held(), "EM: rcu read lock needed\n");
+	lockdep_assert(rcu_read_lock_any_held());
 
 	if (!sum_util)
 		return 0;
@@ -267,7 +267,7 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
 	 * Find the lowest performance state of the Energy Model above the
 	 * requested performance.
 	 */
-	em_table = rcu_dereference(pd->em_table);
+	em_table = rcu_dereference_all(pd->em_table);
 	i = em_pd_get_efficient_state(em_table->state, pd, max_util);
 	ps = &em_table->state[i];
 
-- 
2.43.0