Re: [PATCH 4/8] sched/eevdf: Decay positive lag of sleeping entities

[Peter Zijlstra]

On Tue, Sep 22, 2026 at 02:56:56PM +0200, Vincent Guittot wrote:
> On Tue, 22 Sept 2026 at 12:02, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Mon, Sep 21, 2026 at 05:22:34PM +0200, Vincent Guittot wrote:

> > > +
> > > +     vlag -= calc_delta_fair(delta_exec, se);
> >
> > Should this not be 'W+w' at the very least?, ideally it would be the
> > complete sum of all decaying weight rather than just 'w', but that might
> > be a tad tricky.
> 
> I spent some time thinking about this. The right solution should be
> the one you suggested in your next reply: keeping the sleeping tasks
> "enqueued" in another tree and computing a zero decay vruntime.
> However, I was afraid of the overhead of managing this new tree and
> taking the lock of another rq to dequeue the task. 

Yeah :/

> Anything else in
> between will be an approximation because W at enqueue doesn't reflect
> what happened during the sleep period: The CPU could have been idle
> the entire time, 

Ah but if we let idle reset all the lags, by sequence number or anything
else, then that case doesn't matter.

> but several tasks wake up simultaneously so the 1st
> enqueued will not see a W whereas the other one will.

Right. So this approximation is under estimating, it is the absolute
lowest possible decay time, which seems somewhat unfortunate. OTOH I
agree that keeping the whole second tree and all that comes with that,
might be a tad much.

Still it would be good to find a more reasonable approach. Perhaps using
load_avg ? I think over-estimating it might be a little safer than
under-estimating in this case.