Subject: Re: [PATCH 13/33] sched/isolation: Convert housekeeping cpumasks to rcu pointers

From: Simon Horman <[email-masked]>
Date: Wed, 7 Jan 2026 11:56:53 +0000

On Thu, Jan 01, 2026 at 11:13:38PM +0100, Frederic Weisbecker wrote:
> HK_TYPE_DOMAIN's cpumask will soon be made modifiable by cpuset.
> A synchronization mechanism is then needed to synchronize the updates
> with the housekeeping cpumask readers.
> 
> Turn the housekeeping cpumasks into RCU pointers. Once a housekeeping
> cpumask will be modified, the update side will wait for an RCU grace
> period and propagate the change to interested subsystem when deemed
> necessary.
> 
> Signed-off-by: Frederic Weisbecker <[email-masked]>
> ---
>  kernel/sched/isolation.c | 58 +++++++++++++++++++++++++---------------
>  kernel/sched/sched.h     |  1 +
>  2 files changed, 37 insertions(+), 22 deletions(-)
> 
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 11a623fa6320..83be49ec2b06 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -21,7 +21,7 @@ DEFINE_STATIC_KEY_FALSE(housekeeping_overridden);
>  EXPORT_SYMBOL_GPL(housekeeping_overridden);
>  
>  struct housekeeping {
> -	cpumask_var_t cpumasks[HK_TYPE_MAX];
> +	struct cpumask __rcu *cpumasks[HK_TYPE_MAX];
>  	unsigned long flags;
>  };
>  
> @@ -33,17 +33,28 @@ bool housekeeping_enabled(enum hk_type type)
>  }
>  EXPORT_SYMBOL_GPL(housekeeping_enabled);
>  
> +const struct cpumask *housekeeping_cpumask(enum hk_type type)
> +{
> +	if (static_branch_unlikely(&housekeeping_overridden)) {
> +		if (housekeeping.flags & BIT(type)) {
> +			return rcu_dereference_check(housekeeping.cpumasks[type], 1);
> +		}
> +	}
> +	return cpu_possible_mask;
> +}
> +EXPORT_SYMBOL_GPL(housekeeping_cpumask);
> +

Hi Frederic,

I think this patch should also update the access to housekeeping.cpumasks
in housekeeping_setup(), on line 200, to use housekeeping_cpumask().

As is, sparse flags __rcu a annotation miss match there.

  kernel/sched/isolation.c:200:80: warning: incorrect type in argument 3 (different address spaces)
  kernel/sched/isolation.c:200:80:    expected struct cpumask const *srcp3
  kernel/sched/isolation.c:200:80:    got struct cpumask [noderef] __rcu *

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