Re: [PATCH updated v2] sched/fair: core wide cfs task priority comparison

Peter Zijlstra
2020-05-08T09:09:47Z

On Fri, May 08, 2020 at 04:44:19PM +0800, Aaron Lu wrote:
> On Wed, May 06, 2020 at 04:35:06PM +0200, Peter Zijlstra wrote:

> > Aside from this being way to complicated for what it does -- you
> > could've saved the min_vruntime for each rq and compared them with
> > subtraction -- it is also terminally broken afaict.
> >
> > Consider any infeasible weight scenario. Take for instance two tasks,
> > each bound to their respective sibling, one with weight 1 and one with
> > weight 2. Then the lower weight task will run ahead of the higher weight
> > task without bound.
>
> I don't follow how this could happen. Even the lower weight task runs
> first, after some time, the higher weight task will get its turn and
> from then on, the higher weight task will get more chance to run(due to
> its higher weight and thus, slower accumulation of vruntime).

That seems to assume they're mutually exclusive. In that case, as I
argued, we only have a single runqueue and then yes it works. But if
they're not exclusive, and can run concurrently, it comes apart.