Re: [PATCH v2 1/1] sched/fair: Track current se's EEVDF parameters

[Yiwei Lin]

On 11/3/23 14:45, Abel Wu wrote:
> On 11/2/23 10:20 PM, Yiwei Lin Wrote:
> > After dequeuing the current-picked scheduling entity with
> > `__dequeue_entity`, its contribution to the EEVDF parameters
> > cfs_rq->avg_vruntime and cfs_rq->avg_load are also removed.
> > Because these should in fact be considered for the EEVDF algorithm,
> > we took curr as the special case and inserted back the contributions
> > when requests for cfs_rq->avg_vruntime and cfs_rq->avg_load.
> >
> > Functions like `entity_eligible` which is called insied a loop may
> > therefore recalculate these statistics repeatly and require more effort.
> > Instead, we could just avoid to remove these statistics from
> > cfs_rq->avg_vruntime and cfs_rq->avg_load directly.
> >
> > Signed-off-by: Yiwei Lin <s921975628@xxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 39 +++++++++++----------------------------
> >   1 file changed, 11 insertions(+), 28 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 876798824..a10a73603 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -655,17 +655,9 @@ void avg_vruntime_update(struct cfs_rq *cfs_rq, 
> > s64 delta)
> >    */
> >   u64 avg_vruntime(struct cfs_rq *cfs_rq)
> >   {
> > -    struct sched_entity *curr = cfs_rq->curr;
> >       s64 avg = cfs_rq->avg_vruntime;
> >       long load = cfs_rq->avg_load;
> >   -    if (curr && curr->on_rq) {
> > -        unsigned long weight = scale_load_down(curr->load.weight);
> > -
> > -        avg += entity_key(cfs_rq, curr) * weight;
> > -        load += weight;
> > -    }
> > -
> >       if (load) {
> >           /* sign flips effective floor / ceil */
> >           if (avg < 0)
> > @@ -722,17 +714,9 @@ static void update_entity_lag(struct cfs_rq 
> > *cfs_rq, struct sched_entity *se)
> >    */
> >   int entity_eligible(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >   {
> > -    struct sched_entity *curr = cfs_rq->curr;
> >       s64 avg = cfs_rq->avg_vruntime;
> >       long load = cfs_rq->avg_load;
> >   -    if (curr && curr->on_rq) {
> > -        unsigned long weight = scale_load_down(curr->load.weight);
> > -
> > -        avg += entity_key(cfs_rq, curr) * weight;
> > -        load += weight;
> > -    }
> > -
> >       return avg >= entity_key(cfs_rq, se) * load;
> >   }
> >   @@ -821,11 +805,12 @@ static void __enqueue_entity(struct cfs_rq 
> > *cfs_rq, struct sched_entity *se)
> >                   __entity_less, &min_deadline_cb);
> >   }
> >   -static void __dequeue_entity(struct cfs_rq *cfs_rq, struct 
> > sched_entity *se)
> > +static void __dequeue_entity(struct cfs_rq *cfs_rq, struct 
> > sched_entity *se, bool on_rq)
>
> Please don't add such complexity. Since avg_vruntime includes the whole
> cfs_rq after this change, you can simply avg_vruntime_add() when doing
> enqueue_entity() and avg_vruntime_sub() in dequeue_entity().
Right. I should consider that we'll also use __enqueue_entity to put 
back the 'curr', and
avg_vruntime_add() when doing enqueue_entity() and avg_vruntime_sub() in 
dequeue_entity()
as you mentioned could not only simplify the implementation but also 
address to this issue.
I'll fix it.

> >   {
> >       rb_erase_augmented_cached(&se->run_node, &cfs_rq->tasks_timeline,
> >                     &min_deadline_cb);
> > -    avg_vruntime_sub(cfs_rq, se);
> > +    if (!on_rq)
> > +        avg_vruntime_sub(cfs_rq, se);
> >   }
> >     struct sched_entity *__pick_first_entity(struct cfs_rq *cfs_rq)
> > @@ -1137,6 +1122,7 @@ static void update_curr(struct cfs_rq *cfs_rq)
> >       struct sched_entity *curr = cfs_rq->curr;
> >       u64 now = rq_clock_task(rq_of(cfs_rq));
> >       u64 delta_exec;
> > +    u64 delta_fair;
> >         if (unlikely(!curr))
> >           return;
> > @@ -1158,7 +1144,9 @@ static void update_curr(struct cfs_rq *cfs_rq)
> >       curr->sum_exec_runtime += delta_exec;
> >       schedstat_add(cfs_rq->exec_clock, delta_exec);
> >   -    curr->vruntime += calc_delta_fair(delta_exec, curr);
> > +    delta_fair = calc_delta_fair(delta_exec, curr);
> > +    curr->vruntime += delta_fair;
> > +    cfs_rq->avg_vruntime += delta_fair * 
> > scale_load_down(curr->load.weight);
>
> What if curr->load.weight changes in this time slice?

The change on reweight_entity now don't separate curr and other 
entities. If curr->load.weight is changed,
reweight_entity will do:
1. update_curr() to account the latest contribution of vruntime with old 
weight.
2. avg_vruntime_sub() to remove the curr vruntime with old weight
3. avg_vruntime_add() to insert the curr vruntime with new weight
Although step1 and step2 could be merged together when re-weighting the 
curr entity, but it
seems just find to general implement reweight_entity.