Re: [Patch v3 3/6] sched/fair: Implement prefer sibling imbalance calculation between asymmetric groups

[Tobias Huschle]

On 2023-07-14 15:14, Shrikanth Hegde wrote:
> On 7/8/23 4:27 AM, Tim Chen wrote:
> > From: Tim C Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> >
> > In the current prefer sibling load balancing code, there is an 
> > implicit
> > assumption that the busiest sched group and local sched group are
> > equivalent, hence the tasks to be moved is simply the difference in
> > number of tasks between the two groups (i.e. imbalance) divided by 
> > two.
> >
> > However, we may have different number of cores between the cluster 
> > groups,
> > say when we take CPU offline or we have hybrid groups.  In that case,
> > we should balance between the two groups such that #tasks/#cores ratio
> > is the same between the same between both groups.  Hence the imbalance
>
> nit: type here. the same between is repeated.
>
> > computed will need to reflect this.
> >
> > Adjust the sibling imbalance computation to take into account of the
> > above considerations.
> >
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > ---
> >  kernel/sched/fair.c | 41 +++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 37 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index f636d6c09dc6..f491b94908bf 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9372,6 +9372,41 @@ static inline bool smt_balance(struct lb_env 
> > *env, struct sg_lb_stats *sgs,
> >  	return false;
> >  }
> >
> > +static inline long sibling_imbalance(struct lb_env *env,
> > +				    struct sd_lb_stats *sds,
> > +				    struct sg_lb_stats *busiest,
> > +				    struct sg_lb_stats *local)
> > +{
> > +	int ncores_busiest, ncores_local;
> > +	long imbalance;
>
> can imbalance be unsigned int or unsigned long? as sum_nr_running is
> unsigned int.
>
> > +
> > +	if (env->idle == CPU_NOT_IDLE || !busiest->sum_nr_running)
> > +		return 0;
> > +
> > +	ncores_busiest = sds->busiest->cores;
> > +	ncores_local = sds->local->cores;
> > +
> > +	if (ncores_busiest == ncores_local) {
> > +		imbalance = busiest->sum_nr_running;
> > +		lsub_positive(&imbalance, local->sum_nr_running);
> > +		return imbalance;
> > +	}
> > +
> > +	/* Balance such that nr_running/ncores ratio are same on both groups 
> > */
> > +	imbalance = ncores_local * busiest->sum_nr_running;
> > +	lsub_positive(&imbalance, ncores_busiest * local->sum_nr_running);
> > +	/* Normalize imbalance and do rounding on normalization */
> > +	imbalance = 2 * imbalance + ncores_local + ncores_busiest;
> > +	imbalance /= ncores_local + ncores_busiest;
> > +
>
> Could this work for case where number of CPU/cores would differ
> between two sched groups in a sched domain? Such as problem pointed
> by tobias on S390. It would be nice if this patch can work for that 
> case
> as well. Ran numbers for a few cases. It looks to work.
> https://lore.kernel.org/lkml/20230704134024.GV4253@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#rb0a7dcd28532cafc24101e1d0aed79e6342e3901


Just stumbled upon this patch series as well. In this version it looks
similar to the prototypes I played around with, but more complete.
So I'm happy that my understanding of the load balancer was kinda 
correct :)

From a functional perspective this appears to address the issues we saw 
on s390.

> > +	/* Take advantage of resource in an empty sched group */
> > +	if (imbalance == 0 && local->sum_nr_running == 0 &&
> > +	    busiest->sum_nr_running > 1)
> > +		imbalance = 2;
> > +
>
> I don't see how this case would be true. When there are unequal number
> of cores and local->sum_nr_ruuning
> is 0, and busiest->sum_nr_running is atleast 2, imbalance will be 
> atleast 1.
>
>
> Reviewed-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxxxxxxx>
>
> > +	return imbalance;
> > +}
> > +
> >  static inline bool
> >  sched_reduced_capacity(struct rq *rq, struct sched_domain *sd)
> >  {
> > @@ -10230,14 +10265,12 @@ static inline void 
> > calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> >  		}
> >
> >  		if (busiest->group_weight == 1 || sds->prefer_sibling) {
> > -			unsigned int nr_diff = busiest->sum_nr_running;
> >  			/*
> >  			 * When prefer sibling, evenly spread running tasks on
> >  			 * groups.
> >  			 */
> >  			env->migration_type = migrate_task;
> > -			lsub_positive(&nr_diff, local->sum_nr_running);
> > -			env->imbalance = nr_diff;
> > +			env->imbalance = sibling_imbalance(env, sds, busiest, local);
> >  		} else {
> >
> >  			/*
> > @@ -10424,7 +10457,7 @@ static struct sched_group 
> > *find_busiest_group(struct lb_env *env)
> >  	 * group's child domain.
> >  	 */
> >  	if (sds.prefer_sibling && local->group_type == group_has_spare &&
> > -	    busiest->sum_nr_running > local->sum_nr_running + 1)
> > +	    sibling_imbalance(env, &sds, busiest, local) > 1)
> >  		goto force_balance;
> >
> >  	if (busiest->group_type != group_overloaded) {