Re: [PATCH] sched/fair: fix case with reduced capacity CPU

[Dietmar Eggemann]

On 02/07/2022 06:52, Vincent Guittot wrote:
> The capacity of the CPU available for CFS tasks can be reduced because of
> other activities running on the latter. In such case, it's worth trying to
> move CFS tasks on a CPU with more available capacity.
> 
> The rework of the load balance has filterd the case when the CPU is
> classified to be fully busy but its capacity is reduced.
> 
> Check if CPU's capacity is reduced while gathering load balance statistics
> and classify it group_misfit_task instead of group_fully_busy so we can
> try to move the load on another CPU.
> 
> Reported-by: David Chen <david.chen@xxxxxxxxxxx>
> Reported-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
> 
> David, Zhang,
> 
> I haven't put your tested-by because I have reworked and cleaned the patch to
> cover more cases.
> 
> Could you run some tests with this version ?
> 
> Thanks
> 
>  kernel/sched/fair.c | 50 ++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 40 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a78d2e3b9d49..126b82ef4279 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8798,6 +8798,19 @@ sched_asym(struct lb_env *env, struct sd_lb_stats *sds,  struct sg_lb_stats *sgs
>  	return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
>  }
>  
> +static inline bool
> +sched_reduced_capacity(struct rq *rq, struct sched_domain *sd)
> +{
> +	/*
> +	 * When there is more than 1 task, the group_overloaded case already
> +	 * takes care of cpu with reduced capacity
> +	 */
> +	if (rq->cfs.h_nr_running != 1)
> +		return false;
> +
> +	return check_cpu_capacity(rq, sd);
> +}
> +
>  /**
>   * update_sg_lb_stats - Update sched_group's statistics for load balancing.
>   * @env: The load balancing environment.
> @@ -8820,8 +8833,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  
>  	for_each_cpu_and(i, sched_group_span(group), env->cpus) {
>  		struct rq *rq = cpu_rq(i);
> +		unsigned long load = cpu_load(rq);
>  
> -		sgs->group_load += cpu_load(rq);
> +		sgs->group_load += load;
>  		sgs->group_util += cpu_util_cfs(i);
>  		sgs->group_runnable += cpu_runnable(rq);
>  		sgs->sum_h_nr_running += rq->cfs.h_nr_running;
> @@ -8851,11 +8865,17 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  		if (local_group)
>  			continue;
>  
> -		/* Check for a misfit task on the cpu */
> -		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
> -		    sgs->group_misfit_task_load < rq->misfit_task_load) {
> -			sgs->group_misfit_task_load = rq->misfit_task_load;
> -			*sg_status |= SG_OVERLOAD;
> +		if (env->sd->flags & SD_ASYM_CPUCAPACITY) {
> +			/* Check for a misfit task on the cpu */
> +			if (sgs->group_misfit_task_load < rq->misfit_task_load) {
> +				sgs->group_misfit_task_load = rq->misfit_task_load;
> +				*sg_status |= SG_OVERLOAD;
> +			}
> +		} else if ((env->idle != CPU_NOT_IDLE) &&
> +			   sched_reduced_capacity(rq, env->sd) &&
> +			   (sgs->group_misfit_task_load < load)) {
> +			/* Check for a task running on a CPU with reduced capacity */
> +			sgs->group_misfit_task_load = load;
>  		}
>  	}
>  
> @@ -8908,7 +8928,8 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  	 * CPUs in the group should either be possible to resolve
>  	 * internally or be covered by avg_load imbalance (eventually).
>  	 */
> -	if (sgs->group_type == group_misfit_task &&
> +	if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
> +	    (sgs->group_type == group_misfit_task) &&
>  	    (!capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) ||
>  	     sds->local_stat.group_type != group_has_spare))
>  		return false;
> @@ -9517,9 +9538,18 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>  	busiest = &sds->busiest_stat;
>  
>  	if (busiest->group_type == group_misfit_task) {
> -		/* Set imbalance to allow misfit tasks to be balanced. */
> -		env->migration_type = migrate_misfit;
> -		env->imbalance = 1;
> +		if (env->sd->flags & SD_ASYM_CPUCAPACITY) {
> +			/* Set imbalance to allow misfit tasks to be balanced. */
> +			env->migration_type = migrate_misfit;
> +			env->imbalance = 1;
> +		} else {
> +			/*
> +			 * Set load imbalance to allow moving task from cpu
> +			 * with reduced capacity
> +			 */
> +			env->migration_type = migrate_load;
> +			env->imbalance = busiest->group_misfit_task_load;

I'm wondering why you've chosen that hybrid approach `group_misfit_task
-> migrate_load` and not `group_misfit_task -> migrate_misfit`.

It looks like this `rq->cfs.h_nr_running = 1` case almost (since we
check `busiest->nr_running > 1`) always ends up in the load_balance()
`if (!ld_moved)` condition and need_active_balance() can return 1 in
case `if ((env->idle != CPU_NOT_IDLE) && ...` condition. This leads to
active load_balance and this

IMHO, the same you can achieve when you would stay with
`group_misfit_task -> migrate_misfit`.

I think cpu_load(rq) can be used instead of `rq->misfit_task_load` in
the migrate_misfit case of find_busiest_queue() too.

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