Re: [PATCH -tip 09/32] sched/fair: Snapshot the min_vruntime of CPUs on force idle

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 24, 2020 at 10:31:49AM +1100, Balbir Singh wrote:
> On Mon, Nov 23, 2020 at 07:31:31AM -0500, Vineeth Pillai wrote:
> > Hi Balbir,
> > 
> > On 11/22/20 6:44 AM, Balbir Singh wrote:
> > > 
> > > This seems cumbersome, is there no way to track the min_vruntime via
> > > rq->core->min_vruntime?
> > Do you mean to have a core wide min_vruntime? We had a
> > similar approach from v3 to v7 and it had major issues which
> > broke the assumptions of cfs. There were some lengthy
> > discussions and Peter explained in-depth about the issues:
> > 
> > https://lwn.net/ml/linux-kernel/20200506143506.GH5298@hirez.programming.kicks-ass.net/
> > https://lwn.net/ml/linux-kernel/20200515103844.GG2978@hirez.programming.kicks-ass.net/
> >
> 
> One of the equations in the link is
> 
> ">From which immediately follows that:
> 
>           T_k + T_l
>   S_k+l = ---------                                       (13)
>           W_k + W_l
> 
> On which we can define a combined lag:
> 
>   lag_k+l(i) := S_k+l - s_i                               (14)
> 
> And that gives us the tools to compare tasks across a combined runqueue.
> "
> 
> S_k+l reads like rq->core->vruntime, but it sounds like the equivalent
> of rq->core->vruntime is updated when we enter forced idle as opposed to
> all the time.

Yes, but actually computing and maintaining it is hella hard. Try it
with the very first example in that email (the infeasible weight
scenario) and tell me how it works for you ;-)

Also note that the text below (6) mentions dynamic, then look up the
EEVDF paper which describes some of the dynamics -- the paper is
incomplete and contains a bug, I forget if it ever got updated or if
there's another paper that completes it (the BQF I/O scheduler started
from that and fixed it).

I'm not saying it cannot be done, I'm just saying it is really rather
involved and probably not worth it.

The basic observation the current approach relies on is that al that
faffery basically boils down to the fact that vruntime only means
something when there is contention. And that only the progression is
important not the actual value. That is, this is all fundamentally a
differential equation and our integration constant is meaningless (also
embodied in (7)).

Also, I think the code as proposed here relies on SMT2 and is buggered
for SMT3+. Now, that second link above describes means of making SMT3+
work, but we're not there yet.