[PATCH v4 1/4] man/man2/prctl.2, man/man2const/PR_FUTEX_HASH.2const: Document PR_FUTEX_HASH

Sebastian Andrzej Siewior
2025-06-02T14:01:09Z

The prctl(PR_FUTEX_HASH) is queued for the v6.16 merge window.
Add some documentation of the interface.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
man/man2/prctl.2 | 3 +
man/man2const/PR_FUTEX_HASH.2const | 89 ++++++++++++++++++++++++++++++
2 files changed, 92 insertions(+)
create mode 100644 man/man2const/PR_FUTEX_HASH.2const

diff --git a/man/man2/prctl.2 b/man/man2/prctl.2
index cb5e75bf79ab2..ddfd1d1f5b940 100644
--- a/man/man2/prctl.2
+++ b/man/man2/prctl.2
@@ -150,6 +150,8 @@ with a significance depending on the first one.
.B PR_GET_MDWE
.TQ
.B PR_RISCV_SET_ICACHE_FLUSH_CTX
+.TQ
+.B PR_FUTEX_HASH
.SH RETURN VALUE
On success,
a nonnegative value is returned.
@@ -262,4 +264,5 @@ so these operations should be used with care.
.BR PR_SET_MDWE (2const),
.BR PR_GET_MDWE (2const),
.BR PR_RISCV_SET_ICACHE_FLUSH_CTX (2const),
+.BR PR_FUTEX_HASH (2const),
.BR core (5)
diff --git a/man/man2const/PR_FUTEX_HASH.2const b/man/man2const/PR_FUTEX_HASH.2const
new file mode 100644
index 0000000000000..08511b18335b2
--- /dev/null
+++ b/man/man2const/PR_FUTEX_HASH.2const
@@ -0,0 +1,89 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH PR_FUTEX_HASH 2const (date) "Linux man-pages (unreleased)"
+.SH NAME
+PR_FUTEX_HASH
+\-
+configure the private futex hash
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <linux/prctl.h>" " /* Definition of " PR_* " constants */"
+.B #include <sys/prctl.h>
+.P
+.BI "int prctl(PR_FUTEX_HASH, unsigned long " op ", ...);"
+.fi
+.SH DESCRIPTION
+Configure the attributes for the underlying hash used by the
+.BR futex (2)
+family of operations.
+The Linux kernel uses a hash to distribute the unrelated
+.BR futex (2)
+requests to different data structures
+in order to reduce the lock contention.
+Unrelated requests are requests which are not related to one another
+because they use a different futex word.
+The data structure holds the in-kernel representation of the operation and
+keeps track of the current users which are enqueued and wait for a wake up.
+It also provides synchronisation of waiters against wakers.
+The size of the global hash is determined at boot time
+and is based on the number of CPUs in the system.
+Due to hash collision,
+two unrelated
+.BR futex (2)
+requests can share the same hash bucket.
+This in turn can lead to delays of the
+.BR futex (2)
+operation due to lock contention while accessing the data structure.
+These delays can be problematic on a real-time system
+since random processes can
+share in-kernel locks
+and it is not deterministic which process will be involved.
+.P
+Linux 6.16 implements a process-wide private hash which is used by all
+.BR futex (2)
+operations that specify the
+.B FUTEX_PRIVATE_FLAG
+option as part of the operation.
+Without any configuration
+the kernel will allocate 16 hash slots
+once the first thread has been created.
+If the process continues to create threads,
+the kernel will try to resize the private hash based on the number of threads
+and available CPUs in the system.
+The kernel will only increase the size and will make sure it does not exceed
+the size of the global hash.
+.P
+The user can configure the size of the private
+hash which will also disable the
+automatic resize provided by the kernel.
+.P
+The value in
+.I op
+is one of the options below.
+.TP
+.B PR_FUTEX_HASH_GET_IMMUTABLE
+.TQ
+.B PR_FUTEX_HASH_GET_SLOTS
+.TQ
+.B PR_FUTEX_HASH_SET_SLOTS
+.SH RETURN VALUE
+On success,
+these calls return a nonnegative value.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 6.16.
+.SH SEE ALSO
+.BR prctl (2),
+.BR futex (2),
+.BR PR_FUTEX_HASH_GET_IMMUTABLE (2const),
+.BR PR_FUTEX_HASH_GET_SLOTS (2const),
+.BR PR_FUTEX_HASH_SET_SLOTS (2const)
--
2.49.0