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

[David Laight]

On Mon, 10 Feb 2025 14:44:32 -0500
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> On Mon, 10 Feb 2025 12:27:00 -0500
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> > On Mon, 10 Feb 2025 17:20:59 +0000
> > David Laight <david.laight.linux@xxxxxxxxx> wrote:
> >   
> > > Hmmm... you lose big-time anyway.
> > > 
> > > All you need is a lot of network traffic 'pinch' the process context until
> > > the hardware interrupt, NAPI softint code and rcu softint code completes.
> > > That can easily take several milliseconds.    
> > 
> > Not on PREEMPT_RT. All that runs as threads.
> > 
> > And this is a feature that we would like on RT for non RT tasks.  
> 
> Actually, this doesn't even need PREEMPT_RT to not hit your example. You
> can build and boot your system with interrupts as threads, and that also
> includes softirqs.

And then you lose lots of receive ethernet packets unless you change
all the thread priorities.
(I don't recall anything that makes them run at a low FIFO prioriry.)

And don't mention the mess that happens if you have hardware that is
raising an interrupt ever 10ms and really needs the ISR to run within 10ms.
Someone tried to do that into a VM, the interrupts turn up like busses.
None come for ages and then three arrive together :-)

	David