git: ee44a57e34a8 - main - umtx: use a distribution-fair multiplier for the chain hash

The branch main has been updated by nprice:

URL: https://cgit.FreeBSD.org/src/commit/?id=ee44a57e34a8812b4641d5c9202255ea7883764d

commit ee44a57e34a8812b4641d5c9202255ea7883764d
Author:     Nick Price <nprice@FreeBSD.org>
AuthorDate: 2026-08-31 23:47:12 +0000
Commit:     Nick Price <nprice@FreeBSD.org>
CommitDate: 2026-08-31 23:47:24 +0000

    umtx: use a distribution-fair multiplier for the chain hash
    
      umtxq_hash() multiplies the key by 0x9E370001 and keeps the high bits.  That
      constant is 0x9E37 * 2^16 + 1, so it degenerates for keys whose spacing carries
      trailing zero bits: at a 64 KiB stride it puts 128 of 512 parked waiters onto a
      single chain mutex, and at 16 KiB and up it uses only a handful of the 512
      chains.  Base-system consumers never hit this because libthr places its own wait
      words 128 bytes apart, but a Linux-ABI runtime waiting on addresses it allocates
      itself lands squarely on the floor.  Switch to 0x61C88647, which leaves at most 3
      waiters per chain at the same stride; Linux made this exact change in 2016, after
      judging the sparse constants "actively bad for hashing".
    
    Approved by:    adrian (mentor)
    Reviewed by:    kib, adrian, emaste
    Differential Revision:  https://reviews.freebsd.org/D58337
    
    Signed-off-by: Nick Price <nprice@FreeBSD.org>
---
 sys/kern/kern_umtx.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/sys/kern/kern_umtx.c b/sys/kern/kern_umtx.c
index 69ea90ce9972..ac2469379690 100644
--- a/sys/kern/kern_umtx.c
+++ b/sys/kern/kern_umtx.c
@@ -113,7 +113,7 @@
 			  (td)->td_user_pri <= PRI_MAX_TIMESHARE) ?\
 			 PRI_MAX_TIMESHARE : (td)->td_user_pri)
 
-#define	GOLDEN_RATIO_PRIME	2654404609U
+#define	GOLDEN_RATIO_32		1640531527U
 #ifndef	UMTX_CHAINS
 #define	UMTX_CHAINS		512
 #endif
@@ -380,7 +380,7 @@ umtxq_hash(struct umtx_key *key)
 	unsigned n;
 
 	n = (uintptr_t)key->info.both.a + key->info.both.b;
-	key->hash = ((n * GOLDEN_RATIO_PRIME) >> UMTX_SHIFTS) % UMTX_CHAINS;
+	key->hash = ((n * GOLDEN_RATIO_32) >> UMTX_SHIFTS) % UMTX_CHAINS;
 }
 
 struct umtxq_chain *