Re: [PATCH v3 02/10] sched/fair: Credit a persistent, queue-depth-scaled vlag margin

[Vincent Guittot]

On Fri, 12 Jun 2026 at 08:07, K Prateek Nayak <kprateek.nayak@xxxxxxx> wrote:
>
> Hello Wanpeng,
>
> On 6/12/2026 7:03 AM, Wanpeng Li wrote:
> > -     /* Already eligible: nothing to do. */
> > -     if (vlag >= 0)
> > -             return;
> > +     /* Clamp the margin to entity_lag()'s bound so place_entity() keeps it. */
> > +     max_slice = cfs_rq_max_slice(cfs_rq) + TICK_NSEC;
> > +     lag_limit = calc_delta_fair(max_slice, se);
> > +     margin = eevdf_persistent_margin(cfs_rq, se);
> > +     if (lag_limit && margin > lag_limit)
> > +             margin = lag_limit;
> > +     if (vlag >= 0) {
> > +             if ((u64)vlag >= margin)
> > +                     return;
> > +             want = margin - (u64)vlag;
>
> And we are making an eligible task more eligible but *where* is this
> credit coming from? Someone should be paying the price right?
>
> Just adding an arbitrary credit and then moving the task doing the
> yield_to() 0-lag point is simply not fair IMO.

I agree, you can't arbitrarily add some lag to an entity just because
you want it to run.

>
> > +     } else {
> > +             want = margin + (u64)(-vlag);
> > +     }
>
> --
> Thanks and Regards,
> Prateek
>