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

[Tobias Huschle <huschle@linux.ibm.com>]

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) {