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

[Waiman Long <llong@redhat.com>]

On 10/31/25 12:17 PM, Frederic Weisbecker wrote:
> Le Tue, Oct 21, 2025 at 12:03:05AM -0400, Waiman Long a écrit :
>> 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.
> But this is called within bh_lru_lock() which should have either disabled
> IRQs or preemption off. I would expect rcu_dereference_check() to automatically
> verify those implied RCU read-side critical sections.
>
> Let's see, lockdep_assert_in_rcu_reader() checks preemptible(), which is:
>
> #define preemptible()	(preempt_count() == 0 && !irqs_disabled())
>
> Ah but if !CONFIG_PREEMPT_COUNT:
>
> #define preemptible()	0
>
> Chen did you have !CONFIG_PREEMPT_COUNT ?
>
> Probably lockdep_assert_in_rcu_reader() should be fixed accordingly and consider
> preemption always disabled whenever !CONFIG_PREEMPT_COUNT. Let me check that...

Yes, !CONFIG_PREEMPT_COUNT could be the problem here. I thought it was 
missing rcu_read_lock(), but I didn't really check the code to ensure that.

Cheers, Longman