Re: [RFC][PATCH 1/2] sched: Extended scheduler time slice

[Steven Rostedt]

On Wed, 5 Feb 2025 00:09:51 -0500
Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:

> On Tue, Feb 4, 2025 at 10:03 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > On Tue, 4 Feb 2025 19:56:09 -0500
> > Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> >  
> > > > Here is the RFC I had sent that Peter is referring  
> > >
> > > FWIW, I second the idea of a new syscall for this than (ab)using rseq
> > > and also independence from preemption method. I agree that something
> > > generic is better than relying on preemption method.  
> >
> > So you are for adding another user/kernel memory mapped section?  
> 
> I don't personally mind that.

I'm glad you don't personally mind it. Are you going to help maintain
another memory mapped section?

> 
> > And you are also OK with allowing any task to make an RT task wait longer?
> >
> > Putting my RT hat back on, I would definitely disable that on any system
> > that requires RT.  
> 
> Just so I understand, you are basically saying that you want this
> feature only for FAIR tasks, and allowing RT tasks to extend time
> slice might actually hurt the latency of (other) RT tasks on the
> system right? This assumes PREEMPT_RT because the latency is 50us
> right?

RT tasks don't have a time slice. They are affected by events. An external
interrupt coming in, or a timer going off that states something is
happening. Perhaps we could use this for SCHED_RR or maybe even
SCHED_DEADLINE, as those do have time slices.

But if it does get used, it should only be used when the task being
scheduled is the same SCHED_RR priority, or if SCHED_DEADLINE will not fail
its guarantees.

> 
> But in a poorly designed system, if you have RT tasks at higher
> priority that preempt things lower in RT, that would already cause
> latency anyway. Similarly, I would also consider any PREEMPT_RT system

And that would be a poorly designed system, and not the problem of the
kernel.

> that (mis)uses this API in an RT task as also a poorly designed
> system. I think PREEMPT_RT systems generally require careful design
> anyway.  So the fact that a system is poorly designed and thus causes
> latency is not the kernel's problem IMO.

Correct. And why I don't think this should be used for RT. It's SCHED_OTHER
that doesn't have any control of the sched tick, where this hint can help.

> 
> In any case, if you want this to only work on FAIR tasks and not RT
> tasks, why is that only possible to do with rseq() + LAZY preemption
> and not Prakash's new API + all preemption modes?
> 
> Also you can just ignore RT tasks (not that I'm saying that's a good
> idea but..) in taskshrd_delay_resched() in that patch if you ever
> wanted to do that.
> 
> I just feel the RT latency thing is a non-issue AFAICS.

Have you worked on any RT projects before?

-- Steve