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

Frederic Weisbecker
2025-10-13T20:33:33Z

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>
--
2.51.0