* [PATCH v2 2/6] futex: Use RCU-based per-CPU reference counting instead of rcuref_t
[not found] <20250710110011.384614-1-bigeasy@linutronix.de>
@ 2025-07-10 11:00 ` Sebastian Andrzej Siewior
2025-07-30 12:20 ` André Draszik
0 siblings, 1 reply; 4+ messages in thread
From: Sebastian Andrzej Siewior @ 2025-07-10 11:00 UTC (permalink / raw)
To: linux-kernel
Cc: André Almeida, Darren Hart, Davidlohr Bueso, Ingo Molnar,
Juri Lelli, Peter Zijlstra, Thomas Gleixner, Valentin Schneider,
Waiman Long, Andrew Morton, David Hildenbrand, Liam R. Howlett,
Lorenzo Stoakes, Michal Hocko, Mike Rapoport, Suren Baghdasaryan,
Vlastimil Babka, linux-mm, Sebastian Andrzej Siewior
From: Peter Zijlstra <peterz@infradead.org>
The use of rcuref_t for reference counting introduces a performance bottleneck
when accessed concurrently by multiple threads during futex operations.
Replace rcuref_t with special crafted per-CPU reference counters. The
lifetime logic remains the same.
The newly allocate private hash starts in FR_PERCPU state. In this state, each
futex operation that requires the private hash uses a per-CPU counter (an
unsigned int) for incrementing or decrementing the reference count.
When the private hash is about to be replaced, the per-CPU counters are
migrated to a atomic_t counter mm_struct::futex_atomic.
The migration process:
- Waiting for one RCU grace period to ensure all users observe the
current private hash. This can be skipped if a grace period elapsed
since the private hash was assigned.
- futex_private_hash::state is set to FR_ATOMIC, forcing all users to
use mm_struct::futex_atomic for reference counting.
- After a RCU grace period, all users are guaranteed to be using the
atomic counter. The per-CPU counters can now be summed up and added to
the atomic_t counter. If the resulting count is zero, the hash can be
safely replaced. Otherwise, active users still hold a valid reference.
- Once the atomic reference count drops to zero, the next futex
operation will switch to the new private hash.
call_rcu_hurry() is used to speed up transition which otherwise might be
delay with RCU_LAZY. There is nothing wrong with using call_rcu(). The
side effects would be that on auto scaling the new hash is used later
and the SET_SLOTS prctl() will block longer.
[bigeasy: commit description + mm get/ put_async]
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: linux-mm@kvack.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
include/linux/futex.h | 16 +--
include/linux/mm_types.h | 5 +
include/linux/sched/mm.h | 2 +-
init/Kconfig | 4 -
kernel/fork.c | 8 +-
kernel/futex/core.c | 243 ++++++++++++++++++++++++++++++++++++---
6 files changed, 243 insertions(+), 35 deletions(-)
diff --git a/include/linux/futex.h b/include/linux/futex.h
index b37193653e6b5..9e9750f049805 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -85,18 +85,12 @@ int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)
#ifdef CONFIG_FUTEX_PRIVATE_HASH
int futex_hash_allocate_default(void);
void futex_hash_free(struct mm_struct *mm);
-
-static inline void futex_mm_init(struct mm_struct *mm)
-{
- RCU_INIT_POINTER(mm->futex_phash, NULL);
- mm->futex_phash_new = NULL;
- mutex_init(&mm->futex_hash_lock);
-}
+int futex_mm_init(struct mm_struct *mm);
#else /* !CONFIG_FUTEX_PRIVATE_HASH */
static inline int futex_hash_allocate_default(void) { return 0; }
-static inline void futex_hash_free(struct mm_struct *mm) { }
-static inline void futex_mm_init(struct mm_struct *mm) { }
+static inline int futex_hash_free(struct mm_struct *mm) { return 0; }
+static inline int futex_mm_init(struct mm_struct *mm) { return 0; }
#endif /* CONFIG_FUTEX_PRIVATE_HASH */
#else /* !CONFIG_FUTEX */
@@ -118,8 +112,8 @@ static inline int futex_hash_allocate_default(void)
{
return 0;
}
-static inline void futex_hash_free(struct mm_struct *mm) { }
-static inline void futex_mm_init(struct mm_struct *mm) { }
+static inline int futex_hash_free(struct mm_struct *mm) { return 0; }
+static inline int futex_mm_init(struct mm_struct *mm) { return 0; }
#endif
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index d6b91e8a66d6d..0f0662157066a 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1070,6 +1070,11 @@ struct mm_struct {
struct mutex futex_hash_lock;
struct futex_private_hash __rcu *futex_phash;
struct futex_private_hash *futex_phash_new;
+ /* futex-ref */
+ unsigned long futex_batches;
+ struct rcu_head futex_rcu;
+ atomic_long_t futex_atomic;
+ unsigned int __percpu *futex_ref;
#endif
unsigned long hiwater_rss; /* High-watermark of RSS usage */
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index b13474825130f..2201da0afecc5 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -140,7 +140,7 @@ static inline bool mmget_not_zero(struct mm_struct *mm)
/* mmput gets rid of the mappings and all user-space */
extern void mmput(struct mm_struct *);
-#ifdef CONFIG_MMU
+#if defined(CONFIG_MMU) || defined(CONFIG_FUTEX_PRIVATE_HASH)
/* same as above but performs the slow path from the async context. Can
* be called from the atomic context as well
*/
diff --git a/init/Kconfig b/init/Kconfig
index 666783eb50abd..af4c2f0854554 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1716,13 +1716,9 @@ config FUTEX_PI
depends on FUTEX && RT_MUTEXES
default y
-#
-# marked broken for performance reasons; gives us one more cycle to sort things out.
-#
config FUTEX_PRIVATE_HASH
bool
depends on FUTEX && !BASE_SMALL && MMU
- depends on BROKEN
default y
config FUTEX_MPOL
diff --git a/kernel/fork.c b/kernel/fork.c
index 1ee8eb11f38ba..0b885dcbde9af 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1046,7 +1046,6 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
RCU_INIT_POINTER(mm->exe_file, NULL);
mmu_notifier_subscriptions_init(mm);
init_tlb_flush_pending(mm);
- futex_mm_init(mm);
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !defined(CONFIG_SPLIT_PMD_PTLOCKS)
mm->pmd_huge_pte = NULL;
#endif
@@ -1061,6 +1060,9 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
mm->def_flags = 0;
}
+ if (futex_mm_init(mm))
+ goto fail_mm_init;
+
if (mm_alloc_pgd(mm))
goto fail_nopgd;
@@ -1090,6 +1092,8 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
fail_noid:
mm_free_pgd(mm);
fail_nopgd:
+ futex_hash_free(mm);
+fail_mm_init:
free_mm(mm);
return NULL;
}
@@ -1145,7 +1149,7 @@ void mmput(struct mm_struct *mm)
}
EXPORT_SYMBOL_GPL(mmput);
-#ifdef CONFIG_MMU
+#if defined(CONFIG_MMU) || defined(CONFIG_FUTEX_PRIVATE_HASH)
static void mmput_async_fn(struct work_struct *work)
{
struct mm_struct *mm = container_of(work, struct mm_struct,
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 90d53fb0ee9e1..1dcb4c8a2585d 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -42,7 +42,6 @@
#include <linux/fault-inject.h>
#include <linux/slab.h>
#include <linux/prctl.h>
-#include <linux/rcuref.h>
#include <linux/mempolicy.h>
#include <linux/mmap_lock.h>
@@ -65,7 +64,7 @@ static struct {
#define futex_queues (__futex_data.queues)
struct futex_private_hash {
- rcuref_t users;
+ int state;
unsigned int hash_mask;
struct rcu_head rcu;
void *mm;
@@ -129,6 +128,12 @@ static struct futex_hash_bucket *
__futex_hash(union futex_key *key, struct futex_private_hash *fph);
#ifdef CONFIG_FUTEX_PRIVATE_HASH
+static bool futex_ref_get(struct futex_private_hash *fph);
+static bool futex_ref_put(struct futex_private_hash *fph);
+static bool futex_ref_is_dead(struct futex_private_hash *fph);
+
+enum { FR_PERCPU = 0, FR_ATOMIC };
+
static inline bool futex_key_is_private(union futex_key *key)
{
/*
@@ -142,15 +147,14 @@ bool futex_private_hash_get(struct futex_private_hash *fph)
{
if (fph->immutable)
return true;
- return rcuref_get(&fph->users);
+ return futex_ref_get(fph);
}
void futex_private_hash_put(struct futex_private_hash *fph)
{
- /* Ignore return value, last put is verified via rcuref_is_dead() */
if (fph->immutable)
return;
- if (rcuref_put(&fph->users))
+ if (futex_ref_put(fph))
wake_up_var(fph->mm);
}
@@ -243,14 +247,18 @@ static bool __futex_pivot_hash(struct mm_struct *mm,
fph = rcu_dereference_protected(mm->futex_phash,
lockdep_is_held(&mm->futex_hash_lock));
if (fph) {
- if (!rcuref_is_dead(&fph->users)) {
+ if (!futex_ref_is_dead(fph)) {
mm->futex_phash_new = new;
return false;
}
futex_rehash_private(fph, new);
}
- rcu_assign_pointer(mm->futex_phash, new);
+ new->state = FR_PERCPU;
+ scoped_guard(rcu) {
+ mm->futex_batches = get_state_synchronize_rcu();
+ rcu_assign_pointer(mm->futex_phash, new);
+ }
kvfree_rcu(fph, rcu);
return true;
}
@@ -289,9 +297,7 @@ struct futex_private_hash *futex_private_hash(void)
if (!fph)
return NULL;
- if (fph->immutable)
- return fph;
- if (rcuref_get(&fph->users))
+ if (futex_private_hash_get(fph))
return fph;
}
futex_pivot_hash(mm);
@@ -1527,16 +1533,219 @@ static void futex_hash_bucket_init(struct futex_hash_bucket *fhb,
#define FH_IMMUTABLE 0x02
#ifdef CONFIG_FUTEX_PRIVATE_HASH
+
+/*
+ * futex-ref
+ *
+ * Heavily inspired by percpu-rwsem/percpu-refcount; not reusing any of that
+ * code because it just doesn't fit right.
+ *
+ * Dual counter, per-cpu / atomic approach like percpu-refcount, except it
+ * re-initializes the state automatically, such that the fph swizzle is also a
+ * transition back to per-cpu.
+ */
+
+static void futex_ref_rcu(struct rcu_head *head);
+
+static void __futex_ref_atomic_begin(struct futex_private_hash *fph)
+{
+ struct mm_struct *mm = fph->mm;
+
+ /*
+ * The counter we're about to switch to must have fully switched;
+ * otherwise it would be impossible for it to have reported success
+ * from futex_ref_is_dead().
+ */
+ WARN_ON_ONCE(atomic_long_read(&mm->futex_atomic) != 0);
+
+ /*
+ * Set the atomic to the bias value such that futex_ref_{get,put}()
+ * will never observe 0. Will be fixed up in __futex_ref_atomic_end()
+ * when folding in the percpu count.
+ */
+ atomic_long_set(&mm->futex_atomic, LONG_MAX);
+ smp_store_release(&fph->state, FR_ATOMIC);
+
+ call_rcu_hurry(&mm->futex_rcu, futex_ref_rcu);
+}
+
+static void __futex_ref_atomic_end(struct futex_private_hash *fph)
+{
+ struct mm_struct *mm = fph->mm;
+ unsigned int count = 0;
+ long ret;
+ int cpu;
+
+ /*
+ * Per __futex_ref_atomic_begin() the state of the fph must be ATOMIC
+ * and per this RCU callback, everybody must now observe this state and
+ * use the atomic variable.
+ */
+ WARN_ON_ONCE(fph->state != FR_ATOMIC);
+
+ /*
+ * Therefore the per-cpu counter is now stable, sum and reset.
+ */
+ for_each_possible_cpu(cpu) {
+ unsigned int *ptr = per_cpu_ptr(mm->futex_ref, cpu);
+ count += *ptr;
+ *ptr = 0;
+ }
+
+ /*
+ * Re-init for the next cycle.
+ */
+ this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
+
+ /*
+ * Add actual count, subtract bias and initial refcount.
+ *
+ * The moment this atomic operation happens, futex_ref_is_dead() can
+ * become true.
+ */
+ ret = atomic_long_add_return(count - LONG_MAX - 1, &mm->futex_atomic);
+ if (!ret)
+ wake_up_var(mm);
+
+ WARN_ON_ONCE(ret < 0);
+ mmput_async(mm);
+}
+
+static void futex_ref_rcu(struct rcu_head *head)
+{
+ struct mm_struct *mm = container_of(head, struct mm_struct, futex_rcu);
+ struct futex_private_hash *fph = rcu_dereference_raw(mm->futex_phash);
+
+ if (fph->state == FR_PERCPU) {
+ /*
+ * Per this extra grace-period, everybody must now observe
+ * fph as the current fph and no previously observed fph's
+ * are in-flight.
+ *
+ * Notably, nobody will now rely on the atomic
+ * futex_ref_is_dead() state anymore so we can begin the
+ * migration of the per-cpu counter into the atomic.
+ */
+ __futex_ref_atomic_begin(fph);
+ return;
+ }
+
+ __futex_ref_atomic_end(fph);
+}
+
+/*
+ * Drop the initial refcount and transition to atomics.
+ */
+static void futex_ref_drop(struct futex_private_hash *fph)
+{
+ struct mm_struct *mm = fph->mm;
+
+ /*
+ * Can only transition the current fph;
+ */
+ WARN_ON_ONCE(rcu_dereference_raw(mm->futex_phash) != fph);
+ /*
+ * We enqueue at least one RCU callback. Ensure mm stays if the task
+ * exits before the transition is completed.
+ */
+ mmget(mm);
+
+ /*
+ * In order to avoid the following scenario:
+ *
+ * futex_hash() __futex_pivot_hash()
+ * guard(rcu); guard(mm->futex_hash_lock);
+ * fph = mm->futex_phash;
+ * rcu_assign_pointer(&mm->futex_phash, new);
+ * futex_hash_allocate()
+ * futex_ref_drop()
+ * fph->state = FR_ATOMIC;
+ * atomic_set(, BIAS);
+ *
+ * futex_private_hash_get(fph); // OOPS
+ *
+ * Where an old fph (which is FR_ATOMIC) and should fail on
+ * inc_not_zero, will succeed because a new transition is started and
+ * the atomic is bias'ed away from 0.
+ *
+ * There must be at least one full grace-period between publishing a
+ * new fph and trying to replace it.
+ */
+ if (poll_state_synchronize_rcu(mm->futex_batches)) {
+ /*
+ * There was a grace-period, we can begin now.
+ */
+ __futex_ref_atomic_begin(fph);
+ return;
+ }
+
+ call_rcu_hurry(&mm->futex_rcu, futex_ref_rcu);
+}
+
+static bool futex_ref_get(struct futex_private_hash *fph)
+{
+ struct mm_struct *mm = fph->mm;
+
+ guard(rcu)();
+
+ if (smp_load_acquire(&fph->state) == FR_PERCPU) {
+ this_cpu_inc(*mm->futex_ref);
+ return true;
+ }
+
+ return atomic_long_inc_not_zero(&mm->futex_atomic);
+}
+
+static bool futex_ref_put(struct futex_private_hash *fph)
+{
+ struct mm_struct *mm = fph->mm;
+
+ guard(rcu)();
+
+ if (smp_load_acquire(&fph->state) == FR_PERCPU) {
+ this_cpu_dec(*mm->futex_ref);
+ return false;
+ }
+
+ return atomic_long_dec_and_test(&mm->futex_atomic);
+}
+
+static bool futex_ref_is_dead(struct futex_private_hash *fph)
+{
+ struct mm_struct *mm = fph->mm;
+
+ guard(rcu)();
+
+ if (smp_load_acquire(&fph->state) == FR_PERCPU)
+ return false;
+
+ return atomic_long_read(&mm->futex_atomic) == 0;
+}
+
+int futex_mm_init(struct mm_struct *mm)
+{
+ mutex_init(&mm->futex_hash_lock);
+ RCU_INIT_POINTER(mm->futex_phash, NULL);
+ mm->futex_phash_new = NULL;
+ /* futex-ref */
+ atomic_long_set(&mm->futex_atomic, 0);
+ mm->futex_batches = get_state_synchronize_rcu();
+ mm->futex_ref = alloc_percpu(unsigned int);
+ if (!mm->futex_ref)
+ return -ENOMEM;
+ this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
+ return 0;
+}
+
void futex_hash_free(struct mm_struct *mm)
{
struct futex_private_hash *fph;
+ free_percpu(mm->futex_ref);
kvfree(mm->futex_phash_new);
fph = rcu_dereference_raw(mm->futex_phash);
- if (fph) {
- WARN_ON_ONCE(rcuref_read(&fph->users) > 1);
+ if (fph)
kvfree(fph);
- }
}
static bool futex_pivot_pending(struct mm_struct *mm)
@@ -1549,7 +1758,7 @@ static bool futex_pivot_pending(struct mm_struct *mm)
return true;
fph = rcu_dereference(mm->futex_phash);
- return rcuref_is_dead(&fph->users);
+ return futex_ref_is_dead(fph);
}
static bool futex_hash_less(struct futex_private_hash *a,
@@ -1598,11 +1807,11 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
}
}
- fph = kvzalloc(struct_size(fph, queues, hash_slots), GFP_KERNEL_ACCOUNT | __GFP_NOWARN);
+ fph = kvzalloc(struct_size(fph, queues, hash_slots),
+ GFP_KERNEL_ACCOUNT | __GFP_NOWARN);
if (!fph)
return -ENOMEM;
- rcuref_init(&fph->users, 1);
fph->hash_mask = hash_slots ? hash_slots - 1 : 0;
fph->custom = custom;
fph->immutable = !!(flags & FH_IMMUTABLE);
@@ -1645,7 +1854,7 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
* allocated a replacement hash, drop the initial
* reference on the existing hash.
*/
- futex_private_hash_put(cur);
+ futex_ref_drop(cur);
}
if (new) {
--
2.50.0
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