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futex: Allow to make the private hash immutable

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My initial testing showed that:

	perf bench futex hash

reported less operations/sec with private hash. After using the same
amount of buckets in the private hash as used by the global hash then
the operations/sec were about the same.

This changed once the private hash became resizable. This feature added
an RCU section and reference counting via atomic inc+dec operation into
the hot path.
The reference counting can be avoided if the private hash is made
immutable.
Extend PR_FUTEX_HASH_SET_SLOTS by a fourth argument which denotes if the
private should be made immutable. Once set (to true) the a further
resize is not allowed (same if set to global hash).
Add PR_FUTEX_HASH_GET_IMMUTABLE which returns true if the hash can not
be changed.
Update "perf bench" suite.

For comparison, results of "perf bench futex hash -s":
- Xeon CPU E5-2650, 2 NUMA nodes, total 32 CPUs:
  - Before the introducing task local hash
    shared  Averaged 1.487.148 operations/sec (+- 0,53%), total secs = 10
    private Averaged 2.192.405 operations/sec (+- 0,07%), total secs = 10

  - With the series
    shared  Averaged 1.326.342 operations/sec (+- 0,41%), total secs = 10
    -b128   Averaged   141.394 operations/sec (+- 1,15%), total secs = 10
    -Ib128  Averaged   851.490 operations/sec (+- 0,67%), total secs = 10
    -b8192  Averaged   131.321 operations/sec (+- 2,13%), total secs = 10
    -Ib8192 Averaged 1.923.077 operations/sec (+- 0,61%), total secs = 10
    128 is the default allocation of hash buckets.
    8192 was the previous amount of allocated hash buckets.

- Xeon(R) CPU E7-8890 v3, 4 NUMA nodes, total 144 CPUs:
  - Before the introducing task local hash
    shared   Averaged 1.810.936 operations/sec (+- 0,26%), total secs = 20
    private  Averaged 2.505.801 operations/sec (+- 0,05%), total secs = 20

  - With the series
    shared   Averaged 1.589.002 operations/sec (+- 0,25%), total secs = 20
    -b1024   Averaged    42.410 operations/sec (+- 0,20%), total secs = 20
    -Ib1024  Averaged   740.638 operations/sec (+- 1,51%), total secs = 20
    -b65536  Averaged    48.811 operations/sec (+- 1,35%), total secs = 20
    -Ib65536 Averaged 1.963.165 operations/sec (+- 0,18%), total secs = 20
    1024 is the default allocation of hash buckets.
    65536 was the previous amount of allocated hash buckets.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://lore.kernel.org/r/20250416162921.513656-16-bigeasy@linutronix.de
This commit is contained in:
Sebastian Andrzej Siewior 2025-04-16 18:29:15 +02:00 • committed by Peter Zijlstra
parent bd54df5ea7
commit 63e8595c06
2 changed files with 45 additions and 6 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
include/uapi/linux/prctl.h
View file

@ -367,6 +367,8 @@ struct prctl_mm_map {
/* FUTEX hash management */
#define PR_FUTEX_HASH 78
# define PR_FUTEX_HASH_SET_SLOTS 1
# define FH_FLAG_IMMUTABLE (1ULL << 0)
# define PR_FUTEX_HASH_GET_SLOTS 2
# define PR_FUTEX_HASH_GET_IMMUTABLE 3
#endif /* _LINUX_PRCTL_H */

49
kernel/futex/core.c
View file

@ -63,6 +63,7 @@ struct futex_private_hash {
struct rcu_head rcu;
void *mm;
bool custom;
bool immutable;
struct futex_hash_bucket queues[];
};
@ -132,12 +133,16 @@ static inline bool futex_key_is_private(union futex_key *key)
bool futex_private_hash_get(struct futex_private_hash *fph)
{
if (fph->immutable)
return true;
return rcuref_get(&fph->users);
}
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))
wake_up_var(fph->mm);
}
@ -277,6 +282,8 @@ again:
if (!fph)
return NULL;
if (fph->immutable)
return fph;
if (rcuref_get(&fph->users))
return fph;
}
@ -1383,6 +1390,9 @@ static void futex_hash_bucket_init(struct futex_hash_bucket *fhb,
spin_lock_init(&fhb->lock);
}
#define FH_CUSTOM 0x01
#define FH_IMMUTABLE 0x02
#ifdef CONFIG_FUTEX_PRIVATE_HASH
void futex_hash_free(struct mm_struct *mm)
{
@ -1433,10 +1443,11 @@ static bool futex_hash_less(struct futex_private_hash *a,
return false; /* equal */
}
static int futex_hash_allocate(unsigned int hash_slots, bool custom)
static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
{
struct mm_struct *mm = current->mm;
struct futex_private_hash *fph;
bool custom = flags & FH_CUSTOM;
int i;
if (hash_slots && (hash_slots == 1 || !is_power_of_2(hash_slots)))
@ -1447,7 +1458,7 @@ static int futex_hash_allocate(unsigned int hash_slots, bool custom)
*/
scoped_guard(rcu) {
fph = rcu_dereference(mm->futex_phash);
if (fph && !fph->hash_mask) {
if (fph && (!fph->hash_mask || fph->immutable)) {
if (custom)
return -EBUSY;
return 0;
@ -1461,6 +1472,7 @@ static int futex_hash_allocate(unsigned int hash_slots, bool custom)
rcuref_init(&fph->users, 1);
fph->hash_mask = hash_slots ? hash_slots - 1 : 0;
fph->custom = custom;
fph->immutable = !!(flags & FH_IMMUTABLE);
fph->mm = mm;
for (i = 0; i < hash_slots; i++)
@ -1553,7 +1565,7 @@ int futex_hash_allocate_default(void)
if (current_buckets >= buckets)
return 0;
return futex_hash_allocate(buckets, false);
return futex_hash_allocate(buckets, 0);
}
static int futex_hash_get_slots(void)
@ -1567,9 +1579,22 @@ static int futex_hash_get_slots(void)
return 0;
}
static int futex_hash_get_immutable(void)
{
struct futex_private_hash *fph;
guard(rcu)();
fph = rcu_dereference(current->mm->futex_phash);
if (fph && fph->immutable)
return 1;
if (fph && !fph->hash_mask)
return 1;
return 0;
}
#else
static int futex_hash_allocate(unsigned int hash_slots, bool custom)
static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
{
return -EINVAL;
}
@ -1578,23 +1603,35 @@ static int futex_hash_get_slots(void)
{
return 0;
}
static int futex_hash_get_immutable(void)
{
return 0;
}
#endif
int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)
{
unsigned int flags = FH_CUSTOM;
int ret;
switch (arg2) {
case PR_FUTEX_HASH_SET_SLOTS:
if (arg4 != 0)
if (arg4 & ~FH_FLAG_IMMUTABLE)
return -EINVAL;
ret = futex_hash_allocate(arg3, true);
if (arg4 & FH_FLAG_IMMUTABLE)
flags |= FH_IMMUTABLE;
ret = futex_hash_allocate(arg3, flags);
break;
case PR_FUTEX_HASH_GET_SLOTS:
ret = futex_hash_get_slots();
break;
case PR_FUTEX_HASH_GET_IMMUTABLE:
ret = futex_hash_get_immutable();
break;
default:
ret = -EINVAL;
break;