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

From: Frederic Weisbecker <[email-masked]>
Date: Fri, 31 Oct 2025 17:17:50 +0100

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 <[email-masked]>
> > > ---
> > >   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...

Thanks.

-- 
Frederic Weisbecker
SUSE Labs