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

  From: Tim C Chen <tim.c.chen@linux.intel.com>

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
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@linux.intel.com>
---
 kernel/sched/fair.c | 41 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 37 insertions(+), 4 deletions(-)
  

On 7/8/23 4:27 AM, Tim Chen wrote:
> From: Tim C Chen <tim.c.chen@linux.intel.com>
> 
> 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@linux.intel.com>
> ---
>  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@hirez.programming.kicks-ass.net/T/#rb0a7dcd28532cafc24101e1d0aed79e6342e3901



> +	/* 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@linux.vnet.ibm.com>

> +	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) {
  

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@linux.intel.com>
>> 
>> 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@linux.intel.com>
>> ---
>>  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@hirez.programming.kicks-ass.net/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@linux.vnet.ibm.com>
> 
>> +	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) {
  

On Fri, 2023-07-14 at 18:44 +0530, Shrikanth Hegde wrote:
> 
> On 7/8/23 4:27 AM, Tim Chen wrote:
> > From: Tim C Chen <tim.c.chen@linux.intel.com>
> > 
> > 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.

Thanks for catching.

> 
> > 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@linux.intel.com>
> > ---
> >  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.

It could be made unsigned long. 

> 
> > +
> > +	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? 
> 

Yes.  That's the problem I was targeting. 

> 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@hirez.programming.kicks-ass.net/T/#rb0a7dcd28532cafc24101e1d0aed79e6342e3901
> 
> 
> 
> > +	/* 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@linux.vnet.ibm.com>

Thanks for the review.

> 
> > +	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) {
>
  

On Fri, 2023-07-14 at 13:44 -0700, Tim Chen wrote:
> > > 
> > > +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.
> 
> It could be made unsigned long. 
> 
> 
Though in theory the imbalance can be both positive or negative. We are
considering only positive imbalance here as we only pull task to local group
and do not push task from local group.

Tim
  

On Fri, 2023-07-14 at 16:22 +0200, Tobias Huschle wrote:
> 
> > 
> > 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@hirez.programming.kicks-ass.net/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.

Glad that this patch addresses this common issue across architectures.
I did aim to address the asymmetric groups balancing in general.
Peter pointed out this problem that's inherent when he reviewed the first
version of my patchset. 

Tim

> 
> > 
> > 
> > > +	/* 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@linux.vnet.ibm.com>
> > 
> > > +	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) {
  

On Fri, 2023-07-14 at 18:44 +0530, Shrikanth Hegde wrote:
> 
> > +	/* 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. 

I think you are correct.  With at least 2 task in the busiest group,
imbalance will be at least 1.  This is the effect of doing rounding
when adding the (ncores_local + ncores_busy) rounding factor.  

Returning an imbalance value of 1 will not be correct as we
will be dividing imbalance by 2 and we will still not move task
to the empty group as intended. 

So this code should be updated as below:


diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3fc8d3a3bd22..16bf75e6a775 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9400,7 +9400,7 @@ static inline long sibling_imbalance(struct lb_env *env,
        imbalance /= ncores_local + ncores_busiest;
 
        /* Take advantage of resource in an empty sched group */
-       if (imbalance == 0 && local->sum_nr_running == 0 &&
+       if (imbalance <= 1 && local->sum_nr_running == 0 &&
            busiest->sum_nr_running > 1)
                imbalance = 2;
 

Tim