Re: [PATCH 03/23] tick/nohz: Make nohz_full parameter optional

[sashiko-bot@kernel.org]

Sashiko AI review found 1 potential issue(s):
- [High] Booting with `nohz_full` (no argument) silently disables `CONFIG_RCU_NOCB_CPU_DEFAULT_ALL`.
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commit b2af6c59a7f3101f93296e77bcb1e7af45562ea8
Author: Waiman Long <longman@redhat.com>

tick/nohz: Make nohz_full parameter optional

This commit allows the nohz_full boot option to be specified without
any parameters, forcing it to be enabled without any CPUs initially in
the tick_nohz_full_mask.

[ ... ]

> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index b3337c7231ccb..2d06dcb61f379 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1267,7 +1267,7 @@ void __init rcu_init_nohz(void)
>  	struct shrinker * __maybe_unused lazy_rcu_shrinker;
>  
>  #if defined(CONFIG_NO_HZ_FULL)
> -	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> +	if (tick_nohz_full_running)
>  		cpumask = tick_nohz_full_mask;
>  #endif

If the kernel is booted with nohz_full (no arguments), tick_nohz_full_running
will be true but tick_nohz_full_mask will be empty. This causes cpumask to
be assigned a non-NULL pointer to an empty mask.

Will this inadvertently disable CONFIG_RCU_NOCB_CPU_DEFAULT_ALL?

Looking at the code immediately following this change in rcu_init_nohz():

	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) &&
	    !rcu_state.nocb_is_setup && !cpumask)
		cpumask = cpu_possible_mask;

Since cpumask is no longer NULL, this fallback check evaluates to false.
The subsequent code will then OR with the empty tick_nohz_full_mask, resulting
in rcu_nocb_mask remaining empty and disabling RCU callback offloading for
all CPUs at boot.

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Sashiko AI review · https://sashiko.dev/#/patchset/20260421030351.281436-1-longman@redhat.com?part=3