linux-kernel - Re: [patch V4 08/12] rseq: Implement time slice extension enforcement timer

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Message-ID: <20251116232326.9373-1-hdanton@sina.com>
Date: Mon, 17 Nov 2025 07:23:25 +0800
From: Hillf Danton <hdanton@...a.com>
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>
Subject: Re: [patch V4 08/12] rseq: Implement time slice extension enforcement timer

On Sun, 16 Nov 2025 21:51:21 +0100 (CET) Thomas Gleixner wrote:
> If a time slice extension is granted and the reschedule delayed, the kernel
> has to ensure that user space cannot abuse the extension and exceed the
> maximum granted time.
> 
[snip]
> 
>  #ifdef CONFIG_RSEQ_SLICE_EXTENSION
> +struct slice_timer {
> +	struct hrtimer	timer;
> +	void		*cookie;
> +};
> +
> +unsigned int rseq_slice_ext_nsecs __read_mostly = 30 * NSEC_PER_USEC;
> +static DEFINE_PER_CPU(struct slice_timer, slice_timer);
>  DEFINE_STATIC_KEY_TRUE(rseq_slice_extension_key);
>  
> +static enum hrtimer_restart rseq_slice_expired(struct hrtimer *tmr)
> +{
> +	struct slice_timer *st = container_of(tmr, struct slice_timer, timer);
> +
> +	if (st->cookie == current && current->rseq.slice.state.granted) {
> +		rseq_stat_inc(rseq_stats.s_expired);
> +		set_need_resched_current();
> +	}
> +	return HRTIMER_NORESTART;
> +}
Feel free to add a comment for the migrated tasks, because the percpu
slice timer fails to catch them, to specify how to clamp the maximum
granted time.