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

Chen Ridong
2025-10-21T01:46:34Z

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

--
Best regards,
Ridong