[tip: timers/core] posix-timers: Switch to jhash32()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     781764e0b4394fbd8e8eb39195f8a076b60808b3
Gitweb:        https://git.kernel.org/tip/781764e0b4394fbd8e8eb39195f8a076b60808b3
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Sat, 08 Mar 2025 17:48:40 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Thu, 13 Mar 2025 12:07:17 +01:00

posix-timers: Switch to jhash32()

The hash distribution of hash_32() is suboptimal. jhash32() provides a way
better distribution, which evens out the length of the hash bucket lists,
which in turn avoids large outliers in list walk times.

Due to the sparse ID space (thanks CRIU) there is no guarantee that the
timers will be fully evenly distributed over the hash buckets, but the
behaviour is way better than with hash_32() even for randomly sparse ID
spaces.

For a pathological test case with 64 processes creating and accessing
20000 timers each, this results in a runtime reduction of ~10% and a
significantly reduced runtime variation.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20250308155624.279080328@xxxxxxxxxxxxx


---
 kernel/time/posix-timers.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 23f6d8b..0c4cee3 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -11,8 +11,8 @@
  */
 #include <linux/compat.h>
 #include <linux/compiler.h>
-#include <linux/hash.h>
 #include <linux/init.h>
+#include <linux/jhash.h>
 #include <linux/interrupt.h>
 #include <linux/list.h>
 #include <linux/memblock.h>
@@ -47,11 +47,11 @@ struct timer_hash_bucket {
 
 static struct {
 	struct timer_hash_bucket	*buckets;
-	unsigned long			bits;
+	unsigned long			mask;
 } __timer_data __ro_after_init __aligned(2*sizeof(long));
 
 #define timer_buckets	(__timer_data.buckets)
-#define timer_hashbits	(__timer_data.bits)
+#define timer_hashmask	(__timer_data.mask)
 
 static const struct k_clock * const posix_clocks[];
 static const struct k_clock *clockid_to_kclock(const clockid_t id);
@@ -87,7 +87,7 @@ DEFINE_CLASS_IS_COND_GUARD(lock_timer);
 
 static struct timer_hash_bucket *hash_bucket(struct signal_struct *sig, unsigned int nr)
 {
-	return &timer_buckets[hash_32(hash32_ptr(sig) ^ nr, timer_hashbits)];
+	return &timer_buckets[jhash2((u32 *)&sig, sizeof(sig) / sizeof(u32), nr) & timer_hashmask];
 }
 
 static struct k_itimer *posix_timer_by_id(timer_t id)
@@ -1513,7 +1513,7 @@ static int __init posixtimer_init(void)
 	timer_buckets = alloc_large_system_hash("posixtimers", sizeof(*timer_buckets),
 						size, 0, 0, &shift, NULL, size, size);
 	size = 1UL << shift;
-	timer_hashbits = ilog2(size);
+	timer_hashmask = size - 1;
 
 	for (i = 0; i < size; i++) {
 		spin_lock_init(&timer_buckets[i].lock);