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

[Peter Zijlstra]

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);
> +}