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

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, Oct 28 2025 at 09:33, 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.

If a task gets migrated then it can't overstep the share because the
migration is bringing it back into the kernel, schedules it out and
schedules it in on the new CPU. So the whole accounting start over
freshly. That's the same as if the task gets the extension granted, goes
to user space and gets interrupted again. If that interrupt sets
NEED_RESCHED the grant is "revoked" and the timer fires for nothing.

> Since it is a CPU local timer which is HRTIMER_MODE_HARD, from this CPUs
> perspective it is either about to run or it did run. Therefore the
> hrtimer_try_to_cancel() can't return -1 due to
> hrtimer_callback_running() == true.
> If you drop hrtimer_active() check and scoped_guard(irq),
> hrtimer_try_to_cancel() will do the same hrtimer_active() check as you
> have above followed by disable interrupts via lock_hrtimer_base() and
> here hrtimer_callback_running() can't return true because interrupts are
> disabled and the timer can't run on a remote CPU because it is a
> CPU-local timer.
>
> So you avoid a branch to hrtimer_try_to_cancel() if the timer already
> fired.

Yes you are right. Seems I've suffered from brain congestion. Let me
remove it.

Thanks,

        tglx