linux-kernel - Re: [patch V6 07/11] rseq: Implement time slice extension enforcement timer

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Message-ID: <20251218150524.GY3707837@noisy.programming.kicks-ass.net>
Date: Thu, 18 Dec 2025 16:05:24 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>, Jonathan Corbet <corbet@....net>,
	Prakash Sangappa <prakash.sangappa@...cle.com>,
	Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
	Randy Dunlap <rdunlap@...radead.org>,
	Ron Geva <rongevarg@...il.com>, Waiman Long <longman@...hat.com>
Subject: Re: [patch V6 07/11] rseq: Implement time slice extension
 enforcement timer

On Mon, Dec 15, 2025 at 05:52:22PM +0100, Thomas Gleixner wrote:

> V5: Document the slice extension range - PeterZ

> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -1228,6 +1228,14 @@ reboot-cmd (SPARC only)
>  ROM/Flash boot loader. Maybe to tell it what to do after
>  rebooting. ???
>  
> +rseq_slice_extension_nsec
> +=========================
> +
> +A task can request to delay its scheduling if it is in a critical section
> +via the prctl(PR_RSEQ_SLICE_EXTENSION_SET) mechanism. This sets the maximum
> +allowed extension in nanoseconds before scheduling of the task is enforced.
> +Default value is 30000ns (30us). The possible range is 10000ns (10us) to
> +50000ns (50us).

The important bit: we're not going to increase these numbers. If
anything, I would like the default to be 10us and taint the kernel if
you up it.

I also think we want some tracing/tool to find the actual length of the
extension used (min/avg/max etc.). That is the time between the kernel
finding the extension bit set and arming the timer and the slice_yield()
syscall.