Re: [PATCH 1/4] sched/eevdf: Fix vruntime adjustment on reweight

[Yiwei Lin]

On 11/16/23 13:07, Abel Wu wrote:
> On 11/16/23 12:48 PM, Abel Wu Wrote:
> > On 11/15/23 11:36 PM, Yiwei Lin Wrote:
> > > > @@ -3712,8 +3811,17 @@ static void reweight_entity(struct cfs_rq 
> > > > *cfs_rq, struct sched_entity *se,
> > > >       enqueue_load_avg(cfs_rq, se);
> > > >       if (se->on_rq) {
> > > >           update_load_add(&cfs_rq->load, se->load.weight);
> > > > -        if (cfs_rq->curr != se)
> > > > -            avg_vruntime_add(cfs_rq, se);
> > > > +        if (!curr) {
> > > > +            /*
> > > > +             * The entity's vruntime has been adjusted, so let's 
> > > > check
> > > > +             * whether the rq-wide min_vruntime needs updated too. 
> > > > Since
> > > > +             * the calculations above require stable min_vruntime 
> > > > rather
> > > > +             * than up-to-date one, we do the update at the end of 
> > > > the
> > > > +             * reweight process.
> > > > +             */
> > > > +            __enqueue_entity(cfs_rq, se);
> > > > +            update_min_vruntime(cfs_rq);
> > > > +        }
> > > >       }
> > > >   }
> > > Sorry if I am asking stupid question...... It looks like 
> > > reweight_entity() may have chance to change the weight of 
> > > cfs_rq->curr entity, but we'll never update_min_vruntime() when 
> > > reweighting it. Is there any reason that we can skip the 
> > > update_min_vruntime() for this case?
> >
> > No, you are right!
>
> I was intended to update_min_vruntime() if se->on_rq and no matter
> it is curr or not, just as you suggested. But after a second thought
> I wonder if it is necessary to update *NOW*, since we will always
> update_curr() before making any change to cfs_rq. Thoughts?
I lost the fact that we'll update_min_vruntime() every time we 
update_curr(). Because of this fact, we can indeed wait until we need 
the correct min_vruntime and update_min_vruntime() then. The only 
consideration that I came up with is that the sched_debug may not be 
able to reflect the accurate min_vruntime in time. But this may not be a 
big problem.

Further, I have another advanced thought we can remove the 
update_min_vruntime() here in the reweight_entity() directly to save 
more time. The reason that I think this is because min_vruntime is not 
for normalization of vruntime as before which is required on CFS, so we 
will always update_curr() for the latest min_vruntime before using it. 
Also, the update_min_vruntime() in dequeue_entity() may also be removed 
as the reason, i.e. just do update_min_vruntime() in update_curr() to 
simplify. What do you think?

Thanks,
Yiwei Lin