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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 03, 2021 at 12:02:49PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 13, 2021 at 07:04:35PM -0700, Ricardo Neri wrote:
> > @@ -8507,6 +8616,18 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> >  	if (!sgs->sum_h_nr_running)
> >  		return false;
> >  
> > +	/*
> > +	 * @sg may have been tentatively classified as group_asym_packing.
> > +	 * Now that we have sufficient information about @sds.local, reassess
> > +	 * if asym packing migration can be done. Reclassify @sg. The only
> > +	 * possible results are group_has_spare and group_fully_busy.
> > +	 */
> > +	if (sgs->group_type == group_asym_packing &&
> > +	    !asym_can_pull_tasks(env->dst_cpu, sds, sgs, sg)) {
> > +		sgs->group_asym_packing = 0;
> > +		sgs->group_type = group_classify(env->sd->imbalance_pct, sg, sgs);
> > +	}
> 
> So if this really is all about not having sds.local in
> update_sd_lb_stats(), then that seems fixable. Let me haz a try.

How's this then?

---
 kernel/sched/fair.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3bdc41f22909..e9dcbee5b3d9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8437,6 +8437,21 @@ group_type group_classify(unsigned int imbalance_pct,
 	return group_has_spare;
 }
 
+static inline bool
+sched_asym(struct lb_env *env, struct sd_lb_stats *sds, struct sched_group *group)
+{
+	/*
+	 * Because sd->groups starts with the local group, anything that isn't
+	 * the local group will have access to the local state.
+	 */
+	if (group == sds->local)
+		return false;
+
+	/* XXX do magic here */
+
+	return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
+}
+
 /**
  * update_sg_lb_stats - Update sched_group's statistics for load balancing.
  * @env: The load balancing environment.
@@ -8445,6 +8460,7 @@ group_type group_classify(unsigned int imbalance_pct,
  * @sg_status: Holds flag indicating the status of the sched_group
  */
 static inline void update_sg_lb_stats(struct lb_env *env,
+				      struct sd_lb_stats *sds,
 				      struct sched_group *group,
 				      struct sg_lb_stats *sgs,
 				      int *sg_status)
@@ -8453,7 +8469,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 
 	memset(sgs, 0, sizeof(*sgs));
 
-	local_group = cpumask_test_cpu(env->dst_cpu, sched_group_span(group));
+	local_group = group == sds->local;
 
 	for_each_cpu_and(i, sched_group_span(group), env->cpus) {
 		struct rq *rq = cpu_rq(i);
@@ -8498,9 +8514,8 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 
 	/* Check if dst CPU is idle and preferred to this group */
 	if (env->sd->flags & SD_ASYM_PACKING &&
-	    env->idle != CPU_NOT_IDLE &&
-	    sgs->sum_h_nr_running &&
-	    sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu)) {
+	    env->idle != CPU_NOT_IDLE && sgs->sum_h_nr_running &&
+	    sched_asym(env, sds, group)) {
 		sgs->group_asym_packing = 1;
 	}
 
@@ -9016,7 +9031,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 				update_group_capacity(env->sd, env->dst_cpu);
 		}
 
-		update_sg_lb_stats(env, sg, sgs, &sg_status);
+		update_sg_lb_stats(env, sds, sg, sgs, &sg_status);
 
 		if (local_group)
 			goto next_group;