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

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Message-ID: <20251218151833.GZ3707837@noisy.programming.kicks-ass.net>
Date: Thu, 18 Dec 2025 16:18:33 +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:

> +static void rseq_cancel_slice_extension_timer(void)
> +{
> +	struct slice_timer *st = this_cpu_ptr(&slice_timer);
> +
> +	/*
> +	 * st->cookie can be safely read as preemption is disabled and the
> +	 * timer is CPU local.
> +	 *
> +	 * As this is most probably the first expiring timer, the cancel is
> +	 * expensive as it has to reprogram the hardware, but that's less
> +	 * expensive than going through a full hrtimer_interrupt() cycle
> +	 * for nothing.

So I have these hrtick patches that skip some of that reprogramming --
at the cost of causing those spurious interrupts. Overall that was a
win.

Should we look at the cost of a spurious hrtimer interrupt? IIRC each
base will stop at the first iteration if the timer is 'early', which
wasn't that bad.

> +	 * hrtimer_try_to_cancel() is sufficient here as the timer is CPU
> +	 * local and once the hrtimer code disabled interrupts the timer
> +	 * callback cannot be running.
> +	 */
> +	if (st->cookie == current)
> +		hrtimer_try_to_cancel(&st->timer);
> +}