Re: [PATCH 12/33] sched/isolation: Convert housekeeping cpumasks to rcu pointers

[Waiman Long <llong@redhat.com>]

On 10/20/25 9:46 PM, Chen Ridong wrote:
>
> On 2025/10/14 4:31, Frederic Weisbecker wrote:
>> HK_TYPE_DOMAIN's cpumask will soon be made modifyable 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 <frederic@kernel.org>
>> ---
>>   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 8690fb705089..b46c20b5437f 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);
>> +
>>   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());
>> +			cpu = sched_numa_find_closest(housekeeping_cpumask(type), smp_processor_id());
>>   			if (cpu < nr_cpu_ids)
>>   				return cpu;
>>   
>> -			cpu = cpumask_any_and_distribute(housekeeping.cpumasks[type], cpu_online_mask);
>> +			cpu = cpumask_any_and_distribute(housekeeping_cpumask(type), cpu_online_mask);
>>   			if (likely(cpu < nr_cpu_ids))
>>   				return cpu;
>>   			/*
>> @@ -59,28 +70,18 @@ int housekeeping_any_cpu(enum hk_type type)
>>   }
>>   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];
>> -	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]);
>> +			set_cpus_allowed_ptr(t, housekeeping_cpumask(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_cpumask(type));
>>   	return true;
>>   }
>>   EXPORT_SYMBOL_GPL(housekeeping_test_cpu);
>> @@ -96,20 +97,33 @@ void __init housekeeping_init(void)
>>   
>>   	if (housekeeping.flags & HK_FLAG_KERNEL_NOISE)
>>   		sched_tick_offload_init();
>> -
>> +	/*
>> +	 * Realloc with a proper allocator so that any cpumask update
>> +	 * can indifferently free the old version with kfree().
>> +	 */
>>   	for_each_set_bit(type, &housekeeping.flags, HK_TYPE_MAX) {
>> +		struct cpumask *omask, *nmask = kmalloc(cpumask_size(), GFP_KERNEL);
>> +
>> +		if (WARN_ON_ONCE(!nmask))
>> +			return;
>> +
>> +		omask = rcu_dereference(housekeeping.cpumasks[type]);
>> +
>>   		/* We need at least one CPU to handle housekeeping work */
>> -		WARN_ON_ONCE(cpumask_empty(housekeeping.cpumasks[type]));
>> +		WARN_ON_ONCE(cpumask_empty(omask));
>> +		cpumask_copy(nmask, omask);
>> +		RCU_INIT_POINTER(housekeeping.cpumasks[type], nmask);
>> +		memblock_free(omask, cpumask_size());
>>   	}
>>   }
>>   
>>   static void __init housekeeping_setup_type(enum hk_type type,
>>   					   cpumask_var_t housekeeping_staging)
>>   {
>> +	struct cpumask *mask = memblock_alloc_or_panic(cpumask_size(), SMP_CACHE_BYTES);
>>   
>> -	alloc_bootmem_cpumask_var(&housekeeping.cpumasks[type]);
>> -	cpumask_copy(housekeeping.cpumasks[type],
>> -		     housekeeping_staging);
>> +	cpumask_copy(mask, housekeeping_staging);
>> +	RCU_INIT_POINTER(housekeeping.cpumasks[type], mask);
>>   }
>>   
>>   static int __init housekeeping_setup(char *str, unsigned long flags)
>> @@ -162,7 +176,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
>>   
>>   		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX) {
>>   			if (!cpumask_equal(housekeeping_staging,
>> -					   housekeeping.cpumasks[type])) {
>> +					   housekeeping_cpumask(type))) {
>>   				pr_warn("Housekeeping: nohz_full= must match isolcpus=\n");
>>   				goto free_housekeeping_staging;
>>   			}
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 1f5d07067f60..0c0ef8999fd6 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -42,6 +42,7 @@
>>   #include <linux/ktime_api.h>
>>   #include <linux/lockdep_api.h>
>>   #include <linux/lockdep.h>
>> +#include <linux/memblock.h>
>>   #include <linux/minmax.h>
>>   #include <linux/mm.h>
>>   #include <linux/module.h>
> A warning was detected:
>
> =============================
> WARNING: suspicious RCU usage
> 6.17.0-next-20251009-00033-g4444da88969b #808 Not tainted
> -----------------------------
> kernel/sched/isolation.c:60 suspicious rcu_dereference_check() usage!
>
> other info that might help us debug this:
>
>
> rcu_scheduler_active = 2, debug_locks = 1
> 1 lock held by swapper/0/1:
>   #0: ffff888100600ce0 (&type->i_mutex_dir_key#3){++++}-{4:4}, at: walk_compone
>
> stack backtrace:
> CPU: 3 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.17.0-next-20251009-00033-g4
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239
> Call Trace:
>   <TASK>
>   dump_stack_lvl+0x68/0xa0
>   lockdep_rcu_suspicious+0x148/0x1b0
>   housekeeping_cpumask+0xaa/0xb0
>   housekeeping_test_cpu+0x25/0x40
>   find_get_block_common+0x41/0x3e0
>   bdev_getblk+0x28/0xa0
>   ext4_getblk+0xba/0x2d0
>   ext4_bread_batch+0x56/0x170
>   __ext4_find_entry+0x17c/0x410
>   ? lock_release+0xc6/0x290
>   ext4_lookup+0x7a/0x1d0
>   __lookup_slow+0xf9/0x1b0
>   walk_component+0xe0/0x150
>   link_path_walk+0x201/0x3e0
>   path_openat+0xb1/0xb30
>   ? stack_depot_save_flags+0x41e/0xa00
>   do_filp_open+0xbc/0x170
>   ? _raw_spin_unlock_irqrestore+0x2c/0x50
>   ? __create_object+0x59/0x80
>   ? trace_kmem_cache_alloc+0x1d/0xa0
>   ? vprintk_emit+0x2b2/0x360
>   do_open_execat+0x56/0x100
>   alloc_bprm+0x1a/0x200
>   ? __pfx_kernel_init+0x10/0x10
>   kernel_execve+0x4b/0x160
>   kernel_init+0xe5/0x1c0
>   ret_from_fork+0x185/0x1d0
>   ? __pfx_kernel_init+0x10/0x10
>   ret_from_fork_asm+0x1a/0x30
>   </TASK>
> random: crng init done
>
It is because bh_lru_install() of fs/buffer.c calls cpu_is_isolated() 
without holding a rcu_read_lock. Will need to add a rcu_read_lock() there.

Cheers,
Longman