Re: [PATCH 12/33] sched/isolation: Convert housekeeping cpumasks to rcu pointers
From: Waiman Long
Date: Fri Oct 31 2025 - 15:29:34 EST
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:But this is called within bh_lru_lock() which should have either disabled
On 2025/10/14 4:31, Frederic Weisbecker wrote:It is because bh_lru_install() of fs/buffer.c calls cpu_is_isolated()
HK_TYPE_DOMAIN's cpumask will soon be made modifyable by cpuset.A warning was detected:
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@xxxxxxxxxx>
---
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>
=============================
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
without holding a rcu_read_lock. Will need to add a rcu_read_lock() there.
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