Re: [PATCH 01/33] sched/isolation: Remove housekeeping static key

[Waiman Long <llong@redhat.com>]

On 8/29/25 11:47 AM, Frederic Weisbecker wrote:
> The housekeeping static key in its current use is mostly irrelevant.
> Most of the time, a housekeeping function call had already been issued
> before the static call got a chance to be evaluated, defeating the
> initial call optimization purpose.
>
> housekeeping_cpu() is the sole correct user performing the static call
> before the actual slow-path function call. But it's seldom used in
> fast-path.
>
> Finally the static call prevents from synchronizing correctly against
> dynamic updates of the housekeeping cpumasks through cpusets.
>
> Get away with a simple flag test instead.
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
>   include/linux/sched/isolation.h | 25 +++++----
>   kernel/sched/isolation.c        | 90 ++++++++++++++-------------------
>   2 files changed, 55 insertions(+), 60 deletions(-)
>
> diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
> index d8501f4709b5..f98ba0d71c52 100644
> --- a/include/linux/sched/isolation.h
> +++ b/include/linux/sched/isolation.h
> @@ -25,12 +25,22 @@ enum hk_type {
>   };
>   
>   #ifdef CONFIG_CPU_ISOLATION
> -DECLARE_STATIC_KEY_FALSE(housekeeping_overridden);
> +extern unsigned long housekeeping_flags;
> +
>   extern int housekeeping_any_cpu(enum hk_type type);
>   extern const struct cpumask *housekeeping_cpumask(enum hk_type type);
>   extern bool housekeeping_enabled(enum hk_type type);
>   extern void housekeeping_affine(struct task_struct *t, enum hk_type type);
>   extern bool housekeeping_test_cpu(int cpu, enum hk_type type);
> +
> +static inline bool housekeeping_cpu(int cpu, enum hk_type type)
> +{
> +	if (housekeeping_flags & BIT(type))
> +		return housekeeping_test_cpu(cpu, type);
> +	else
> +		return true;
> +}
> +
>   extern void __init housekeeping_init(void);
>   
>   #else
> @@ -58,17 +68,14 @@ static inline bool housekeeping_test_cpu(int cpu, enum hk_type type)
>   	return true;
>   }
>   
> +static inline bool housekeeping_cpu(int cpu, enum hk_type type)
> +{
> +	return true;
> +}
> +
>   static inline void housekeeping_init(void) { }
>   #endif /* CONFIG_CPU_ISOLATION */
>   
> -static inline bool housekeeping_cpu(int cpu, enum hk_type type)
> -{
> -#ifdef CONFIG_CPU_ISOLATION
> -	if (static_branch_unlikely(&housekeeping_overridden))
> -		return housekeeping_test_cpu(cpu, type);
> -#endif
> -	return true;
> -}
>   
>   static inline bool cpu_is_isolated(int cpu)
>   {
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index a4cf17b1fab0..2a6fc6fc46fb 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -16,19 +16,13 @@ enum hk_flags {
>   	HK_FLAG_KERNEL_NOISE	= BIT(HK_TYPE_KERNEL_NOISE),
>   };
>   
> -DEFINE_STATIC_KEY_FALSE(housekeeping_overridden);
> -EXPORT_SYMBOL_GPL(housekeeping_overridden);
> -
> -struct housekeeping {
> -	cpumask_var_t cpumasks[HK_TYPE_MAX];
> -	unsigned long flags;
> -};
> -
> -static struct housekeeping housekeeping;
> +static cpumask_var_t housekeeping_cpumasks[HK_TYPE_MAX];
> +unsigned long housekeeping_flags;

Should we add the__read_mostly attribute to housekeeping_flags to 
prevent possible false cacheline sharing problem?

Other than that, LGTM

Cheers,
Longman

> +EXPORT_SYMBOL_GPL(housekeeping_flags);
>   
>   bool housekeeping_enabled(enum hk_type type)
>   {
> -	return !!(housekeeping.flags & BIT(type));
> +	return !!(housekeeping_flags & BIT(type));
>   }
>   EXPORT_SYMBOL_GPL(housekeeping_enabled);
>   
> @@ -36,50 +30,46 @@ int housekeeping_any_cpu(enum hk_type type)
>   {
>   	int cpu;
>   
> -	if (static_branch_unlikely(&housekeeping_overridden)) {
> -		if (housekeeping.flags & BIT(type)) {
> -			cpu = sched_numa_find_closest(housekeeping.cpumasks[type], smp_processor_id());
> -			if (cpu < nr_cpu_ids)
> -				return cpu;
> +	if (housekeeping_flags & BIT(type)) {
> +		cpu = sched_numa_find_closest(housekeeping_cpumasks[type], smp_processor_id());
> +		if (cpu < nr_cpu_ids)
> +			return cpu;
>   
> -			cpu = cpumask_any_and_distribute(housekeeping.cpumasks[type], cpu_online_mask);
> -			if (likely(cpu < nr_cpu_ids))
> -				return cpu;
> -			/*
> -			 * Unless we have another problem this can only happen
> -			 * at boot time before start_secondary() brings the 1st
> -			 * housekeeping CPU up.
> -			 */
> -			WARN_ON_ONCE(system_state == SYSTEM_RUNNING ||
> -				     type != HK_TYPE_TIMER);
> -		}
> +		cpu = cpumask_any_and_distribute(housekeeping_cpumasks[type], cpu_online_mask);
> +		if (likely(cpu < nr_cpu_ids))
> +			return cpu;
> +		/*
> +		 * Unless we have another problem this can only happen
> +		 * at boot time before start_secondary() brings the 1st
> +		 * housekeeping CPU up.
> +		 */
> +		WARN_ON_ONCE(system_state == SYSTEM_RUNNING ||
> +			     type != HK_TYPE_TIMER);
>   	}
> +
>   	return smp_processor_id();
>   }
>   EXPORT_SYMBOL_GPL(housekeeping_any_cpu);
>   
>   const struct cpumask *housekeeping_cpumask(enum hk_type type)
>   {
> -	if (static_branch_unlikely(&housekeeping_overridden))
> -		if (housekeeping.flags & BIT(type))
> -			return housekeeping.cpumasks[type];
> +	if (housekeeping_flags & BIT(type))
> +		return housekeeping_cpumasks[type];
>   	return cpu_possible_mask;
>   }
>   EXPORT_SYMBOL_GPL(housekeeping_cpumask);
>   
>   void housekeeping_affine(struct task_struct *t, enum hk_type type)
>   {
> -	if (static_branch_unlikely(&housekeeping_overridden))
> -		if (housekeeping.flags & BIT(type))
> -			set_cpus_allowed_ptr(t, housekeeping.cpumasks[type]);
> +	if (housekeeping_flags & BIT(type))
> +		set_cpus_allowed_ptr(t, housekeeping_cpumasks[type]);
>   }
>   EXPORT_SYMBOL_GPL(housekeeping_affine);
>   
>   bool housekeeping_test_cpu(int cpu, enum hk_type type)
>   {
> -	if (static_branch_unlikely(&housekeeping_overridden))
> -		if (housekeeping.flags & BIT(type))
> -			return cpumask_test_cpu(cpu, housekeeping.cpumasks[type]);
> +	if (housekeeping_flags & BIT(type))
> +		return cpumask_test_cpu(cpu, housekeeping_cpumasks[type]);
>   	return true;
>   }
>   EXPORT_SYMBOL_GPL(housekeeping_test_cpu);
> @@ -88,17 +78,15 @@ void __init housekeeping_init(void)
>   {
>   	enum hk_type type;
>   
> -	if (!housekeeping.flags)
> +	if (!housekeeping_flags)
>   		return;
>   
> -	static_branch_enable(&housekeeping_overridden);
> -
> -	if (housekeeping.flags & HK_FLAG_KERNEL_NOISE)
> +	if (housekeeping_flags & HK_FLAG_KERNEL_NOISE)
>   		sched_tick_offload_init();
>   
> -	for_each_set_bit(type, &housekeeping.flags, HK_TYPE_MAX) {
> +	for_each_set_bit(type, &housekeeping_flags, HK_TYPE_MAX) {
>   		/* We need at least one CPU to handle housekeeping work */
> -		WARN_ON_ONCE(cpumask_empty(housekeeping.cpumasks[type]));
> +		WARN_ON_ONCE(cpumask_empty(housekeeping_cpumasks[type]));
>   	}
>   }
>   
> @@ -106,8 +94,8 @@ static void __init housekeeping_setup_type(enum hk_type type,
>   					   cpumask_var_t housekeeping_staging)
>   {
>   
> -	alloc_bootmem_cpumask_var(&housekeeping.cpumasks[type]);
> -	cpumask_copy(housekeeping.cpumasks[type],
> +	alloc_bootmem_cpumask_var(&housekeeping_cpumasks[type]);
> +	cpumask_copy(housekeeping_cpumasks[type],
>   		     housekeeping_staging);
>   }
>   
> @@ -117,7 +105,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
>   	unsigned int first_cpu;
>   	int err = 0;
>   
> -	if ((flags & HK_FLAG_KERNEL_NOISE) && !(housekeeping.flags & HK_FLAG_KERNEL_NOISE)) {
> +	if ((flags & HK_FLAG_KERNEL_NOISE) && !(housekeeping_flags & HK_FLAG_KERNEL_NOISE)) {
>   		if (!IS_ENABLED(CONFIG_NO_HZ_FULL)) {
>   			pr_warn("Housekeeping: nohz unsupported."
>   				" Build with CONFIG_NO_HZ_FULL\n");
> @@ -139,7 +127,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
>   	if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
>   		__cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
>   		__cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask);
> -		if (!housekeeping.flags) {
> +		if (!housekeeping_flags) {
>   			pr_warn("Housekeeping: must include one present CPU, "
>   				"using boot CPU:%d\n", smp_processor_id());
>   		}
> @@ -148,7 +136,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
>   	if (cpumask_empty(non_housekeeping_mask))
>   		goto free_housekeeping_staging;
>   
> -	if (!housekeeping.flags) {
> +	if (!housekeeping_flags) {
>   		/* First setup call ("nohz_full=" or "isolcpus=") */
>   		enum hk_type type;
>   
> @@ -157,26 +145,26 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
>   	} else {
>   		/* Second setup call ("nohz_full=" after "isolcpus=" or the reverse) */
>   		enum hk_type type;
> -		unsigned long iter_flags = flags & housekeeping.flags;
> +		unsigned long iter_flags = flags & housekeeping_flags;
>   
>   		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX) {
>   			if (!cpumask_equal(housekeeping_staging,
> -					   housekeeping.cpumasks[type])) {
> +					   housekeeping_cpumasks[type])) {
>   				pr_warn("Housekeeping: nohz_full= must match isolcpus=\n");
>   				goto free_housekeeping_staging;
>   			}
>   		}
>   
> -		iter_flags = flags & ~housekeeping.flags;
> +		iter_flags = flags & ~housekeeping_flags;
>   
>   		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
>   			housekeeping_setup_type(type, housekeeping_staging);
>   	}
>   
> -	if ((flags & HK_FLAG_KERNEL_NOISE) && !(housekeeping.flags & HK_FLAG_KERNEL_NOISE))
> +	if ((flags & HK_FLAG_KERNEL_NOISE) && !(housekeeping_flags & HK_FLAG_KERNEL_NOISE))
>   		tick_nohz_full_setup(non_housekeeping_mask);
>   
> -	housekeeping.flags |= flags;
> +	housekeeping_flags |= flags;
>   	err = 1;
>   
>   free_housekeeping_staging: