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

Chen Ridong
2025-10-21T01:57:09Z

On 2025/10/21 9:46, 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
>

This warning was likely introduced by patch 13, which added the housekeeping_dereference_check
condition, and is not caused by the current patch.

--
Best regards,
Ridong