Re: [PATCH v2 3/4] sched/fair: Consider SMT in ASYM_PACKING load balance

[Peter Zijlstra]

On Tue, Apr 13, 2021 at 07:04:35PM -0700, Ricardo Neri wrote:
> +static bool cpu_group_is_smt(int cpu, struct sched_group *sg)
> +{
> +#ifdef CONFIG_SCHED_SMT
> +	if (!static_branch_likely(&sched_smt_present))
> +		return false;
> +
> +	if (sg->group_weight == 1)
> +		return false;
> +
> +	return cpumask_equal(sched_group_span(sg), cpu_smt_mask(cpu));
> +#else
> +	return false;
> +#endif
> +}
> +
> +/**
> + * asym_can_pull_tasks - Check whether the load balancing CPU can pull tasks
> + * @dst_cpu:	CPU doing the load balancing
> + * @sds:	Load-balancing data with statistics of the local group
> + * @sgs:	Load-balancing statistics of the candidate busiest group
> + * @sg:		The candidate busiet group
> + *
> + * Check the state of the SMT siblings of both @sds::local and @sg and decide
> + * if @dst_cpu can pull tasks. If @dst_cpu does not have SMT siblings, it can
> + * pull tasks if two or more of the SMT siblings of @sg are busy. If only one
> + * CPU in @sg is busy, pull tasks only if @dst_cpu has higher priority.
> + *
> + * If both @dst_cpu and @sg have SMT siblings. Even the number of idle CPUs
> + * between @sds::local and @sg. Thus, pull tasks from @sg if the difference
> + * between the number of busy CPUs is 2 or more. If the difference is of 1,
> + * only pull if @dst_cpu has higher priority. If @sg does not have SMT siblings
> + * only pull tasks if all of the SMT siblings of @dst_cpu are idle and @sg
> + * has lower priority.
> + */
> +static bool asym_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds,
> +				struct sg_lb_stats *sgs, struct sched_group *sg)
> +{
> +#ifdef CONFIG_SCHED_SMT
> +	int cpu, local_busy_cpus, sg_busy_cpus;
> +	bool local_is_smt, sg_is_smt;
> +
> +	if (!arch_asym_check_smt_siblings())
> +		return true;
> +
> +	cpu = group_first_cpu(sg);
> +	local_is_smt = cpu_group_is_smt(dst_cpu, sds->local);
> +	sg_is_smt = cpu_group_is_smt(cpu, sg);

Would something like this make sense?

---
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8533,21 +8533,6 @@ static inline void update_sg_lb_stats(st
 				sgs->group_capacity;
 }
 
-static bool cpu_group_is_smt(int cpu, struct sched_group *sg)
-{
-#ifdef CONFIG_SCHED_SMT
-	if (!static_branch_likely(&sched_smt_present))
-		return false;
-
-	if (sg->group_weight == 1)
-		return false;
-
-	return cpumask_equal(sched_group_span(sg), cpu_smt_mask(cpu));
-#else
-	return false;
-#endif
-}
-
 /**
  * asym_can_pull_tasks - Check whether the load balancing CPU can pull tasks
  * @dst_cpu:	CPU doing the load balancing
@@ -8578,8 +8563,8 @@ static bool asym_can_pull_tasks(int dst_
 		return true;
 
 	cpu = group_first_cpu(sg);
-	local_is_smt = cpu_group_is_smt(dst_cpu, sds->local);
-	sg_is_smt = cpu_group_is_smt(cpu, sg);
+	local_is_smt = sds->local->flags & SD_SHARE_CPUCAPACITY;
+	sg_is_smt = sg->flags & SD_SHARE_CPUCAPACITY;
 
 	sg_busy_cpus = sgs->group_weight - sgs->idle_cpus;
 
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1795,6 +1795,7 @@ struct sched_group {
 	unsigned int		group_weight;
 	struct sched_group_capacity *sgc;
 	int			asym_prefer_cpu;	/* CPU of highest priority in group */
+	int			flags;
 
 	/*
 	 * The CPUs this group covers.
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -916,10 +916,12 @@ build_group_from_child_sched_domain(stru
 		return NULL;
 
 	sg_span = sched_group_span(sg);
-	if (sd->child)
+	if (sd->child) {
 		cpumask_copy(sg_span, sched_domain_span(sd->child));
-	else
+		sg->flags = sd->child->flags;
+	} else {
 		cpumask_copy(sg_span, sched_domain_span(sd));
+	}
 
 	atomic_inc(&sg->ref);
 	return sg;
@@ -1169,6 +1171,7 @@ static struct sched_group *get_group(int
 	if (child) {
 		cpumask_copy(sched_group_span(sg), sched_domain_span(child));
 		cpumask_copy(group_balance_mask(sg), sched_group_span(sg));
+		sg->flags = child->flags;
 	} else {
 		cpumask_set_cpu(cpu, sched_group_span(sg));
 		cpumask_set_cpu(cpu, group_balance_mask(sg));