From nobody Fri Dec 19 00:22:50 2025 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A21882C327E; Mon, 2 Jun 2025 14:01:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748872871; cv=none; b=coYKb33Uoodi1iSw55+s748KQCcSjxg011LlrPoeBw376g32k4havAVaAF5zhpoBsez+3/Yp+XmOM71anYcsHp/1Cle1XOBf/zIrzxq4Q3CAK+X1dKGDVNOxpV2Y++Ry8V+JQXlXNCP3S3huZv1P7EzOLNR6GerjngxsZwrMvRM= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748872871; c=relaxed/simple; bh=HUcczS7kF+nDU707roRjQQJDLKJgw1PMxAfm7beuSjU=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=nfUYSjCqua42tnYqvM/kImG/IAx4sEfn6JUn3RPco4Za5unDaAoHX3APqkAcPm0ccrXiykdkhrE+0Ju3K0vJ375A1BMUE9YHEmCAgt7/hTW6sZvfgHnnKBHcUyEn5T4a3b3EYo+h73FCziPy0VzkB4umh8HxF2JqAr3M12x/qN0= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=fblVALSm; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=p/Ph2A2x; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="fblVALSm"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="p/Ph2A2x" From: Sebastian Andrzej Siewior DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1748872867; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Owk2aippojDeHXRPl4IzFpvfST1YewhCtUMATmqlZR0=; b=fblVALSm8kKqlhBosy5pnJKptznWE1yB6Fuwp7GnUMy9gLmEQyx00PWpJaBEbckbUpk/xP 24LCjGDwrufbT6scBoGDSfpUcaNbYQC4uA+h4jiT2eyIspbfp3EXWWMTGMnCFpgHuUBFY8 S0CNILw1KwY8NrxysiZOKsA+btsrxDR3CnFhjOrMyIXqgSuLTzA2q+C0QzgOHkFasJoMch mndpyDBpDJ3wKHRlQuPFjaDdM3pAaH1yonwx4bxohXQsLyiAFWKK5VIki/DUfCMcoL++XA xABxsmzy/E8HpMs39wB4fqUK51AruirCdpN3JxqDplLJEL1S7brxFUPSsKQzCA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1748872867; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Owk2aippojDeHXRPl4IzFpvfST1YewhCtUMATmqlZR0=; b=p/Ph2A2x0+B2FWLjTq+Zj2iCNuOQtWCBv0Qmyro4An3Jx+ij6a/Gap16odIMrA0PWDX7XE PeylWAnpU+4Xw5DA== To: linux-kernel@vger.kernel.org, linux-man@vger.kernel.org Cc: Alejandro Colomar , =?UTF-8?q?Andr=C3=A9=20Almeida?= , Darren Hart , Davidlohr Bueso , Ingo Molnar , Juri Lelli , Peter Zijlstra , Thomas Gleixner , Valentin Schneider , Waiman Long , Sebastian Andrzej Siewior Subject: [PATCH v4 1/4] man/man2/prctl.2, man/man2const/PR_FUTEX_HASH.2const: Document PR_FUTEX_HASH Date: Mon, 2 Jun 2025 16:01:01 +0200 Message-ID: <20250602140104.2769223-2-bigeasy@linutronix.de> In-Reply-To: <20250602140104.2769223-1-bigeasy@linutronix.de> References: <20250602140104.2769223-1-bigeasy@linutronix.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 --- 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_HA= SH.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 " " /* Definition of " PR_* " constants */" +.B #include +.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 thre= ads +and available CPUs in the system. +The kernel will only increase the size and will make sure it does not exce= ed +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) --=20 2.49.0