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

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Message-ID: <20260117095737.GG1890602@noisy.programming.kicks-ass.net>
Date: Sat, 17 Jan 2026 10:57:37 +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:

> +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).
+
+This value has a direct correlation to the worst case scheduling latency;
+increment at your own risk.


> +unsigned int rseq_slice_ext_nsecs __read_mostly = 30 * NSEC_PER_USEC;

Changed default to 10us

Also, given the results of that slice_test thing, we might possibly get
away with a much lower value still. 

Prakash, could you possibly capture a trace of hrtimer_start,
hrtimer_cancel and hrtimer_expire_entry for your Oracle workload and run
that python thing on it?


> +#ifdef CONFIG_SYSCTL
> +static const unsigned int rseq_slice_ext_nsecs_min = 10 * NSEC_PER_USEC;
> +static const unsigned int rseq_slice_ext_nsecs_max = 50 * NSEC_PER_USEC;
> +
> +static const struct ctl_table rseq_slice_ext_sysctl[] = {
> +	{
> +		.procname	= "rseq_slice_extension_nsec",
> +		.data		= &rseq_slice_ext_nsecs,
> +		.maxlen		= sizeof(unsigned int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_douintvec_minmax,
> +		.extra1		= (unsigned int *)&rseq_slice_ext_nsecs_min,
> +		.extra2		= (unsigned int *)&rseq_slice_ext_nsecs_max,
> +	},
> +};
> +
> +static void rseq_slice_sysctl_init(void)
> +{
> +	if (rseq_slice_extension_enabled())
> +		register_sysctl_init("kernel", rseq_slice_ext_sysctl);
> +}
> +#else /* CONFIG_SYSCTL */
> +static inline void rseq_slice_sysctl_init(void) { }
> +#endif  /* !CONFIG_SYSCTL */

And I was contemplating moving this to DebugFS rather than sysctl.