Re: [patch V6 10/11] entry: Hook up rseq time slice extension

[Peter Zijlstra]

On Tue, Dec 16, 2025 at 10:37:24AM -0500, Mathieu Desnoyers wrote:
> On 2025-12-15 13:24, Thomas Gleixner wrote:
> > Wire the grant decision function up in exit_to_user_mode_loop()
> > 
> [...]
> > +/* TIF bits, which prevent a time slice extension. */
> > +#ifdef CONFIG_PREEMPT_RT
> > +# define TIF_SLICE_EXT_SCHED	(_TIF_NEED_RESCHED_LAZY)
> > +#else
> > +# define TIF_SLICE_EXT_SCHED	(_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)
> 
> It would be relevant to explain the difference between RT and non-RT
> in the commit message.

So if you include TIF_NEED_RESCHED the extension period directly affects
the minimum scheduler delay like:

  min(extension_period, min_sched_delay)

because this is strictly a from-userspace thing. That is, it is
equivalent to the in-kernel preemption/IRQ disabled regions -- with
exception of the scheduler critical sections itself.

As I've agrued many times -- I don't see a fundamental reason to not do
this for RT -- but perhaps further reduce the magic number such that its
impact cannot be observed on a 'good' machine.

But yes, if/when we do this on RT it needs the promise to agressively
decrease the magic number any time it can actually be measured to impact
performance.

cyclictest should probably get a mode where it (ab)uses the feature to
failure before we do this.

Anyway, I don't mind excluding RT for now, but it *does* deserve a
comment.