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

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Dietmar,

On 3/16/2026 5:06 AM, Dietmar Eggemann wrote:
> We need to adapt em_cpu_energy() in include/linux/energy_model.h as
> well.

I clearly didn't test the EAS bits! I'll make sure to spin something up
with QEMU next time to hit these paths.

> 
> ---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>

Thank you for catching my mistake and for the fix! Feel free to
include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.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];
>  

-- 
Thanks and Regards,
Prateek