Re: [PATCH 14/17] sched/eevdf: Better handle mixed slice length

[Hillf Danton <hdanton@sina.com>]

On 02 Apr 2023 04:40:20 +0200 Mike Galbraith <efault@gmx.de>
> On Sun, 2023-04-02 at 07:23 +0800, Hillf Danton wrote:
> > On 31 Mar 2023 17:26:51 +0200 Vincent Guittot <vincent.guittot@linaro.org>
> > >
> > > I wanted to stress this situation with a simple use case but it seems
> > > that even without changing the slice, there is a fairness problem:
> > >
> > > Task A always run
> > > Task B loops on : running 1ms then sleeping 1ms
> > > default nice and latency nice prio bot both
> > > each task should get around 50% of the time.
> > >
> > > The fairness is ok with tip/sched/core
> > > but with eevdf, Task B only gets around 30%
> >
> > Convincing evidence for glitch in wakeup preempt.
> 
> If you turn on PLACE_BONUS, it'll mimic FAIR_SLEEPERS.. but if you then
> do some testing, you'll probably turn it right back off.
> 
> The 50/50 split in current code isn't really any more fair, as soon as
> you leave the tiny bubble of fairness, it's not the least bit fair.
> Nor is that tiny bubble all rainbows and unicorns, it brought with it
> benchmark wins and losses, like everything that changes more than
> comments, its price being service latency variance.
> 
> The short term split doesn't really mean all that much, some things
> will like the current fair-bubble better, some eevdf virtual deadline
> math and its less spiky service.  We'll see.
> 
> I'm kinda hoping eevdf works out, FAIR_SLEEPERS is quite annoying to
> squabble with.

Yeah no matter whatever role FAIR_SLEEPERS could play next week, this paves
a brick for Vlastimil Babka to take it over leaving Peter happy to sit
back with netflix on.