Re: [PATCH v4 11/11] sched/fair: rework find_idlest_group

[Valentin Schneider <valentin.schneider@arm.com>]

Hi Vincent,

Apologies for the delayed review on that one. I have a few comments inline,
otherwise for the misfit part, if at all still relevant:

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>

On 18/10/2019 14:26, Vincent Guittot wrote:
>  static struct sched_group *
>  find_idlest_group(struct sched_domain *sd, struct task_struct *p,
> +		  int this_cpu, int sd_flag);
                                    ^^^^^^^
That parameter is now unused. AFAICT it was only used to special-case fork
events (sd flag & SD_BALANCE_FORK). I didn't see any explicit handling of
this case in the rework, I assume the new group type classification makes
it possible to forgo?

> @@ -8241,6 +8123,252 @@ static inline enum fbq_type fbq_classify_rq(struct rq *rq)
>  }
>  #endif /* CONFIG_NUMA_BALANCING */
>  
> +
> +struct sg_lb_stats;
> +
> +/*
> + * update_sg_wakeup_stats - Update sched_group's statistics for wakeup.
> + * @denv: The ched_domain level to look for idlest group.
> + * @group: sched_group whose statistics are to be updated.
> + * @sgs: variable to hold the statistics for this group.
> + */
> +static inline void update_sg_wakeup_stats(struct sched_domain *sd,
> +					  struct sched_group *group,
> +					  struct sg_lb_stats *sgs,
> +					  struct task_struct *p)
> +{
> +	int i, nr_running;
> +
> +	memset(sgs, 0, sizeof(*sgs));
> +
> +	for_each_cpu(i, sched_group_span(group)) {
> +		struct rq *rq = cpu_rq(i);
> +
> +		sgs->group_load += cpu_load(rq);
> +		sgs->group_util += cpu_util_without(i, p);
> +		sgs->sum_h_nr_running += rq->cfs.h_nr_running;
> +
> +		nr_running = rq->nr_running;
> +		sgs->sum_nr_running += nr_running;
> +
> +		/*
> +		 * No need to call idle_cpu() if nr_running is not 0
> +		 */
> +		if (!nr_running && idle_cpu(i))
> +			sgs->idle_cpus++;
> +
> +
> +	}
> +
> +	/* Check if task fits in the group */
> +	if (sd->flags & SD_ASYM_CPUCAPACITY &&
> +	    !task_fits_capacity(p, group->sgc->max_capacity)) {
> +		sgs->group_misfit_task_load = 1;
> +	}
> +
> +	sgs->group_capacity = group->sgc->capacity;
> +
> +	sgs->group_type = group_classify(sd->imbalance_pct, group, sgs);
> +
> +	/*
> +	 * Computing avg_load makes sense only when group is fully busy or
> +	 * overloaded
> +	 */
> +	if (sgs->group_type < group_fully_busy)
> +		sgs->avg_load = (sgs->group_load * SCHED_CAPACITY_SCALE) /
> +				sgs->group_capacity;
> +}
> +
> +static bool update_pick_idlest(struct sched_group *idlest,

Nit: could we name this update_sd_pick_idlest() to follow
update_sd_pick_busiest()? It's the kind of thing where if I typed
"update_sd" in gtags I'd like to see both listed, seeing as they are
*very* similar. And we already have update_sg_{wakeup, lb}_stats().

> +			       struct sg_lb_stats *idlest_sgs,
> +			       struct sched_group *group,
> +			       struct sg_lb_stats *sgs)
> +{
> +	if (sgs->group_type < idlest_sgs->group_type)
> +		return true;
> +
> +	if (sgs->group_type > idlest_sgs->group_type)
> +		return false;
> +
> +	/*
> +	 * The candidate and the current idles group are the same type of
> +	 * group. Let check which one is the idlest according to the type.
> +	 */
> +
> +	switch (sgs->group_type) {
> +	case group_overloaded:
> +	case group_fully_busy:
> +		/* Select the group with lowest avg_load. */
> +		if (idlest_sgs->avg_load <= sgs->avg_load)
> +			return false;
> +		break;
> +
> +	case group_imbalanced:
> +	case group_asym_packing:
> +		/* Those types are not used in the slow wakeup path */
> +		return false;
> +
> +	case group_misfit_task:
> +		/* Select group with the highest max capacity */
> +		if (idlest->sgc->max_capacity >= group->sgc->max_capacity)
> +			return false;
> +		break;
> +
> +	case group_has_spare:
> +		/* Select group with most idle CPUs */
> +		if (idlest_sgs->idle_cpus >= sgs->idle_cpus)
> +			return false;
> +		break;
> +	}
> +
> +	return true;
> +}
> +
> +/*
> + * find_idlest_group finds and returns the least busy CPU group within the
> + * domain.
> + *
> + * Assumes p is allowed on at least one CPU in sd.
> + */
> +static struct sched_group *
> +find_idlest_group(struct sched_domain *sd, struct task_struct *p,
> +		  int this_cpu, int sd_flag)
> +{
> +	struct sched_group *idlest = NULL, *local = NULL, *group = sd->groups;
> +	struct sg_lb_stats local_sgs, tmp_sgs;
> +	struct sg_lb_stats *sgs;
> +	unsigned long imbalance;
> +	struct sg_lb_stats idlest_sgs = {
> +			.avg_load = UINT_MAX,
> +			.group_type = group_overloaded,
> +	};
> +
> +	imbalance = scale_load_down(NICE_0_LOAD) *
> +				(sd->imbalance_pct-100) / 100;
> +
> +	do {
> +		int local_group;
> +
> +		/* Skip over this group if it has no CPUs allowed */
> +		if (!cpumask_intersects(sched_group_span(group),
> +					p->cpus_ptr))
> +			continue;
> +
> +		local_group = cpumask_test_cpu(this_cpu,
> +					       sched_group_span(group));
> +
> +		if (local_group) {
> +			sgs = &local_sgs;
> +			local = group;
> +		} else {
> +			sgs = &tmp_sgs;
> +		}
> +
> +		update_sg_wakeup_stats(sd, group, sgs, p);
> +
> +		if (!local_group && update_pick_idlest(idlest, &idlest_sgs, group, sgs)) {
> +			idlest = group;
> +			idlest_sgs = *sgs;
> +		}
> +
> +	} while (group = group->next, group != sd->groups);
> +
> +
> +	/* There is no idlest group to push tasks to */
> +	if (!idlest)
> +		return NULL;
> +
> +	/*
> +	 * If the local group is idler than the selected idlest group
> +	 * don't try and push the task.
> +	 */
> +	if (local_sgs.group_type < idlest_sgs.group_type)
> +		return NULL;
> +
> +	/*
> +	 * If the local group is busier than the selected idlest group
> +	 * try and push the task.
> +	 */
> +	if (local_sgs.group_type > idlest_sgs.group_type)
> +		return idlest;
> +
> +	switch (local_sgs.group_type) {
> +	case group_overloaded:
> +	case group_fully_busy:
> +		/*
> +		 * When comparing groups across NUMA domains, it's possible for
> +		 * the local domain to be very lightly loaded relative to the
> +		 * remote domains but "imbalance" skews the comparison making
> +		 * remote CPUs look much more favourable. When considering
> +		 * cross-domain, add imbalance to the load on the remote node
> +		 * and consider staying local.
> +		 */
> +
> +		if ((sd->flags & SD_NUMA) &&
> +		    ((idlest_sgs.avg_load + imbalance) >= local_sgs.avg_load))
> +			return NULL;
> +
> +		/*
> +		 * If the local group is less loaded than the selected
> +		 * idlest group don't try and push any tasks.
> +		 */
> +		if (idlest_sgs.avg_load >= (local_sgs.avg_load + imbalance))
> +			return NULL;
> +
> +		if (100 * local_sgs.avg_load <= sd->imbalance_pct * idlest_sgs.avg_load)
> +			return NULL;
> +		break;
> +
> +	case group_imbalanced:
> +	case group_asym_packing:
> +		/* Those type are not used in the slow wakeup path */
> +		return NULL;

I suppose group_asym_packing could be handled similarly to misfit, right?
i.e. make the group type group_asym_packing if

  !sched_asym_prefer(sg.asym_prefer_cpu, local.asym_prefer_cpu)

> +
> +	case group_misfit_task:
> +		/* Select group with the highest max capacity */
> +		if (local->sgc->max_capacity >= idlest->sgc->max_capacity)
> +			return NULL;

Got confused a bit here due to the naming; in this case 'group_misfit_task'
only means 'if placed on this group, the task will be misfit'. If the
idlest group will cause us to remain misfit, but can give us some extra
capacity, I think it makes sense to move.

> +		break;
> +
> +	case group_has_spare:
> +		if (sd->flags & SD_NUMA) {
> +#ifdef CONFIG_NUMA_BALANCING
> +			int idlest_cpu;
> +			/*
> +			 * If there is spare capacity at NUMA, try to select
> +			 * the preferred node
> +			 */
> +			if (cpu_to_node(this_cpu) == p->numa_preferred_nid)
> +				return NULL;
> +
> +			idlest_cpu = cpumask_first(sched_group_span(idlest));
> +			if (cpu_to_node(idlest_cpu) == p->numa_preferred_nid)
> +				return idlest;
> +#endif
> +			/*
> +			 * Otherwise, keep the task on this node to stay close
> +			 * its wakeup source and improve locality. If there is
> +			 * a real need of migration, periodic load balance will
> +			 * take care of it.
> +			 */
> +			if (local_sgs.idle_cpus)
> +				return NULL;
> +		}
> +
> +		/*
> +		 * Select group with highest number of idle cpus. We could also
> +		 * compare the utilization which is more stable but it can end
> +		 * up that the group has less spare capacity but finally more
> +		 * idle cpus which means more opportunity to run task.
> +		 */
> +		if (local_sgs.idle_cpus >= idlest_sgs.idle_cpus)
> +			return NULL;
> +		break;
> +	}
> +
> +	return idlest;
> +}
> +
>  /**
>   * update_sd_lb_stats - Update sched_domain's statistics for load balancing.
>   * @env: The load balancing environment.
>