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Message-ID: <20250502180154.GY4439@noisy.programming.kicks-ass.net>
Date: Fri, 2 May 2025 20:01:54 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org,
André Almeida <andrealmeid@...lia.com>,
Darren Hart <dvhart@...radead.org>,
Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Valentin Schneider <vschneid@...hat.com>,
Waiman Long <longman@...hat.com>,
Shrikanth Hegde <sshegde@...ux.ibm.com>
Subject: Re: [PATCH v12 15/21] futex: Allow to make the private hash immutable
On Wed, Apr 16, 2025 at 06:29:15PM +0200, Sebastian Andrzej Siewior wrote:
> 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
> a 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.
Does the below make sense? This changes arg4 into a flags field and uses
bit0 for immutable.
(the point where I got upset is where arg4==2 was accepted :-)
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -367,6 +367,7 @@ 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
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1440,7 +1440,10 @@ static bool futex_hash_less(struct futex
return false; /* equal */
}
-static int futex_hash_allocate(unsigned int hash_slots, unsigned int immutable, bool custom)
+#define FH_CUSTOM 0x01
+#define FH_IMMUTABLE 0x02
+
+static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
{
struct mm_struct *mm = current->mm;
struct futex_private_hash *fph;
@@ -1448,8 +1451,6 @@ static int futex_hash_allocate(unsigned
if (hash_slots && (hash_slots == 1 || !is_power_of_2(hash_slots)))
return -EINVAL;
- if (immutable > 2)
- return -EINVAL;
/*
* Once we've disabled the global hash there is no way back.
@@ -1469,8 +1470,8 @@ static int futex_hash_allocate(unsigned
rcuref_init(&fph->users, 1);
fph->hash_mask = hash_slots ? hash_slots - 1 : 0;
- fph->custom = custom;
- fph->immutable = !!immutable;
+ fph->custom = !!(flags & FH_CUSTOM);
+ fph->immutable = !!(flags & FH_IMMUTABLE);
fph->mm = mm;
for (i = 0; i < hash_slots; i++)
@@ -1563,7 +1564,7 @@ int futex_hash_allocate_default(void)
if (current_buckets >= buckets)
return 0;
- return futex_hash_allocate(buckets, 0, false);
+ return futex_hash_allocate(buckets, 0);
}
static int futex_hash_get_slots(void)
@@ -1592,7 +1593,7 @@ static int futex_hash_get_immutable(void
#else
-static int futex_hash_allocate(unsigned int hash_slots, unsigned int immutable, bool custom)
+static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
{
return -EINVAL;
}
@@ -1610,11 +1611,16 @@ static int futex_hash_get_immutable(void
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:
- ret = futex_hash_allocate(arg3, arg4, true);
+ if (arg4 & ~FH_FLAG_IMMUTABLE)
+ return -EINVAL;
+ if (arg4 & FH_FLAG_IMMUTABLE)
+ flags |= FH_IMMUTABLE;
+ ret = futex_hash_allocate(arg3, flags);
break;
case PR_FUTEX_HASH_GET_SLOTS:
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