linux-kernel - Re: [patch V2 08/12] rseq: Implement time slice extension enforcement timer

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Message-ID: <db7f7264-6ccf-4f55-929a-4c2e813dd8f5@amd.com>
Date: Tue, 28 Oct 2025 14:21:24 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Thomas Gleixner
	<tglx@...utronix.de>
CC: LKML <linux-kernel@...r.kernel.org>, Mathieu Desnoyers
	<mathieu.desnoyers@...icios.com>, Peter Zijlstra <peterz@...radead.org>,
	"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>,
	Steven Rostedt <rostedt@...dmis.org>, Arnd Bergmann <arnd@...db.de>,
	<linux-arch@...r.kernel.org>
Subject: Re: [patch V2 08/12] rseq: Implement time slice extension enforcement
 timer

Hello Sebastian,

On 10/28/2025 2:03 PM, Sebastian Andrzej Siewior wrote:
> On 2025-10-27 17:26:29 [+0100], Thomas Gleixner wrote:
>> On Mon, Oct 27 2025 at 12:38, Sebastian Andrzej Siewior wrote:
>>> On 2025-10-22 14:57:38 [+0200], Thomas Gleixner wrote:
>>>> +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();
>>>> +	}
>>>
>>> You arm the timer while leaving to userland. Once in userland the task
>>> can be migrated to another CPU. Once migrated, this CPU can host another
>>> task while the timer fires and does nothing.
>>
>> That's inevitable. If the scheduler decides to do that then there is
>> nothing which can be done about it and that's why the cookie pointer
>> exists.
> 
> Without an interrupt on the target CPU, there is nothing stopping the
> task from overstepping its fair share.

When the task moves CPU, the rseq_exit_user_update() would clear all
of the slice extension state before running the task again. The task
will start off again with "rseq->slice_ctrl.request" and
"rseq->slice_ctrl.granted" both at 0 signifying the task was
rescheduled.

As for overstepping the limits on the previous CPU, the EEVDF
algorithm (using the task's "vlag" - the vruntime deviation from the
"avg_vruntime") would penalize it accordingly when enqueued.

The previous CPU would just get a spurious interrupt and since the
timer cookie doesn't match with "current", the handler does
nothing and goes away.

-- 
Thanks and Regards,
Prateek