Re: [Patch v3 1/6] sched/fair: Determine active load balance for SMT sched groups

[Shrikanth Hegde]

On 7/15/23 4:35 AM, Tim Chen wrote:
> On Fri, 2023-07-14 at 18:36 +0530, Shrikanth Hegde wrote:
> 
>>
>>
>> If we consider symmetric platforms which have SMT4 such as power10. 
>> we have a topology like below. multiple such MC will form DIE(PKG)
>>
>>
>> [0 2 4 6][1 3 5 7][8 10 12 14][9 11 13 15]
>> [--SMT--][--SMT--][----SMT---][---SMT----]
>> [--sg1--][--sg1--][---sg1----][---sg1----]
>> [--------------MC------------------------]
>>
>> In case of SMT4, if there is any group which has 2 or more tasks, that 
>> group will be marked as group_smt_balance. previously, if that group had 2
>> or 3 tasks, it would have been marked as group_has_spare. Since all the groups have 
>> SMT that means behavior would be same fully busy right? That can cause some 
>> corner cases. No?
> 
> You raised a good point. I was looking from SMT2
> perspective so group_smt_balance implies group_fully_busy.
> That is no longer true for SMT4.
> 
> I am thinking of the following fix on the current patch
> to take care of SMT4. Do you think this addresses

Thanks Tim for taking a look at it again. 

Yes. I think this would address some of the corner cases. 
Any SMT4 group having 2,3,4 will have smt_balance as the group type, and busiest one 
is the one which has least number of idle cpu's. (same conditions as group_has_spare)




> concerns from you and Tobias?
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 294a662c9410..3fc8d3a3bd22 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9588,6 +9588,17 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>                 break;
>  
>         case group_smt_balance:
> +               /* no idle cpus on both groups handled by group_fully_busy below */
> +               if (sgs->idle_cpus != 0 || busiest->idle_cpus != 0) {
> +                       if (sgs->idle_cpus > busiest->idle_cpus)
> +                               return false;
> +                       if (sgs->idle_cpus < busiest->idle_cpus)
> +                               return true;
> +                       if (sgs->sum_nr_running <= busiest_sum_nr_running)
> +                               return false;
> +                       else
> +                               return true;
> +               }
> 
> 
> I will be on vacation next three weeks so my response will be slow.
> 
> Tim
> 
>>

Small suggestion to above code to avoid compiler warning of switch case falling
through and else case can be removed, since update_sd_pick_busiest by default returns true.

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e5a75c76bcaa..ae364ac6f22e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9728,9 +9728,9 @@ static bool update_sd_pick_busiest(struct lb_env *env,
                                return true;
                        if (sgs->sum_nr_running <= busiest->sum_nr_running)
                                return false;
-                       else
-                               return true;
                }
+               break;
+
        case group_fully_busy:
                /*
                 * Select the fully busy group with highest avg_load. In



>> One example is Lets say sg1 has 4 tasks. and sg2 has 0 tasks and is trying to do 
>> load balance. Previously imbalance would have been 2, instead now imbalance would be 1.
>> But in subsequent lb it would be balanced.
>>
>>
>>
>>> +	return false;
>>> +}
>>> +
>>>  static inline bool
>>>  sched_reduced_capacity(struct rq *rq, struct sched_domain *sd)
>>>  {
>>> @@ -9425,6 +9464,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>>>  		sgs->group_asym_packing = 1;
>>>  	}
>>>
>>> +	/* Check for loaded SMT group to be balanced to dst CPU */
>>> +	if (!local_group && smt_balance(env, sgs, group))
>>> +		sgs->group_smt_balance = 1;
>>> +
>>>  	sgs->group_type = group_classify(env->sd->imbalance_pct, group, sgs);
>>>
>>>  	/* Computing avg_load makes sense only when group is overloaded */
>>> @@ -9509,6 +9552,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>>>  			return false;
>>>  		break;
>>>
>>> +	case group_smt_balance:
>>>  	case group_fully_busy:
>>>  		/*
>>>  		 * Select the fully busy group with highest avg_load. In
>>> @@ -9537,6 +9581,18 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>>>  		break;
>>>
>>>  	case group_has_spare:
>>> +		/*
>>> +		 * Do not pick sg with SMT CPUs over sg with pure CPUs,
>>> +		 * as we do not want to pull task off SMT core with one task
>>> +		 * and make the core idle.
>>> +		 */
>>> +		if (smt_vs_nonsmt_groups(sds->busiest, sg)) {
>>> +			if (sg->flags & SD_SHARE_CPUCAPACITY && sgs->sum_h_nr_running <= 1)
>>> +				return false;
>>> +			else
>>> +				return true;> +		}
>>> +
>>>  		/*
>>>  		 * Select not overloaded group with lowest number of idle cpus
>>>  		 * and highest number of running tasks. We could also compare
>>> @@ -9733,6 +9789,7 @@ static bool update_pick_idlest(struct sched_group *idlest,
>>>
>>>  	case group_imbalanced:
>>>  	case group_asym_packing:
>>> +	case group_smt_balance:
>>>  		/* Those types are not used in the slow wakeup path */
>>>  		return false;
>>>
>>> @@ -9864,6 +9921,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>>>
>>>  	case group_imbalanced:
>>>  	case group_asym_packing:
>>> +	case group_smt_balance:
>>>  		/* Those type are not used in the slow wakeup path */
>>>  		return NULL;
>>>
>>> @@ -10118,6 +10176,13 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>>>  		return;
>>>  	}
>>>
>>> +	if (busiest->group_type == group_smt_balance) {
>>> +		/* Reduce number of tasks sharing CPU capacity */
>>> +		env->migration_type = migrate_task;
>>> +		env->imbalance = 1;
>>> +		return;
>>> +	}
>>> +
>>>  	if (busiest->group_type == group_imbalanced) {
>>>  		/*
>>>  		 * In the group_imb case we cannot rely on group-wide averages
>>> @@ -10363,16 +10428,23 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>>>  		goto force_balance;
>>>
>>>  	if (busiest->group_type != group_overloaded) {
>>> -		if (env->idle == CPU_NOT_IDLE)
>>> +		if (env->idle == CPU_NOT_IDLE) {
>>>  			/*
>>>  			 * If the busiest group is not overloaded (and as a
>>>  			 * result the local one too) but this CPU is already
>>>  			 * busy, let another idle CPU try to pull task.
>>>  			 */
>>>  			goto out_balanced;
>>> +		}
>>> +
>>> +		if (busiest->group_type == group_smt_balance &&
>>> +		    smt_vs_nonsmt_groups(sds.local, sds.busiest)) {
>>> +			/* Let non SMT CPU pull from SMT CPU sharing with sibling */
>>> +			goto force_balance;
>>> +		}
>>>
>>>  		if (busiest->group_weight > 1 &&
>>> -		    local->idle_cpus <= (busiest->idle_cpus + 1))
>>> +		    local->idle_cpus <= (busiest->idle_cpus + 1)) {
>>>  			/*
>>>  			 * If the busiest group is not overloaded
>>>  			 * and there is no imbalance between this and busiest
>>> @@ -10383,12 +10455,14 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>>>  			 * there is more than 1 CPU per group.
>>>  			 */
>>>  			goto out_balanced;
>>> +		}
>>>
>>> -		if (busiest->sum_h_nr_running == 1)
>>> +		if (busiest->sum_h_nr_running == 1) {
>>>  			/*
>>>  			 * busiest doesn't have any tasks waiting to run
>>>  			 */
>>>  			goto out_balanced;
>>> +		}
>>>  	}
>>>
>>>  force_balance:
>