Re: [PATCH RFC v1 1/4] rcu/nocb: Consolidate rcu_nocb_cpu_offload/deoffload functions

[Frederic Weisbecker]

Le Tue, Feb 24, 2026 at 06:04:32PM -0500, Joel Fernandes a écrit :
> The rcu_nocb_cpu_offload() and rcu_nocb_cpu_deoffload() functions are
> nearly duplicates.
> 
> Therefore, extract the common logic into rcu_nocb_cpu_toggle_offload()
> which takes an 'offload' boolean, and make both exported functions
> simple wrappers.
> 
> This eliminates a bunch of duplicate code at the call sites, namely
> mutex locking, CPU hotplug locking and CPU online checks.
> 
> Signed-off-by: Joel Fernandes <joelagnelf@xxxxxxxxxx>
> ---
>  kernel/rcu/tree_nocb.h | 89 ++++++++++++++++++++++--------------------
>  1 file changed, 46 insertions(+), 43 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index b3337c7231cc..5b041134d6bb 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1081,30 +1081,6 @@ static int rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
>  	return 0;
>  }
>  
> -int rcu_nocb_cpu_deoffload(int cpu)
> -{
> -	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> -	int ret = 0;
> -
> -	cpus_read_lock();
> -	mutex_lock(&rcu_state.nocb_mutex);
> -	if (rcu_rdp_is_offloaded(rdp)) {
> -		if (!cpu_online(cpu)) {
> -			ret = rcu_nocb_rdp_deoffload(rdp);
> -			if (!ret)
> -				cpumask_clear_cpu(cpu, rcu_nocb_mask);
> -		} else {
> -			pr_info("NOCB: Cannot CB-deoffload online CPU %d\n", rdp->cpu);
> -			ret = -EINVAL;
> -		}
> -	}
> -	mutex_unlock(&rcu_state.nocb_mutex);
> -	cpus_read_unlock();
> -
> -	return ret;
> -}
> -EXPORT_SYMBOL_GPL(rcu_nocb_cpu_deoffload);
> -
>  static bool rcu_nocb_rdp_offload_wait_cond(struct rcu_data *rdp)
>  {
>  	unsigned long flags;
> @@ -1149,27 +1125,54 @@ static int rcu_nocb_rdp_offload(struct rcu_data *rdp)
>  	return 0;
>  }
>  
> +/* Common helper for CPU offload/deoffload operations. */
> +static int rcu_nocb_cpu_toggle_offload(int cpu, bool offload)
> +{
> +	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> +	bool currently_offloaded;
> +	int ret = 0;
> +
> +	cpus_read_lock();
> +	mutex_lock(&rcu_state.nocb_mutex);
> +
> +	currently_offloaded = rcu_rdp_is_offloaded(rdp);

Do we really need that extra variable?

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

-- 
Frederic Weisbecker
SUSE Labs