[PATCH 04/10] sched, fair: Shrink sg_lb_stats and play memset games

[Peter Zijlstra]

We can shrink sg_lb_stats because rq::nr_running is an 'unsigned int'
and cpu numbers are 'int'

Before:
  sgs:        /* size: 72, cachelines: 2, members: 10 */
  sds:        /* size: 184, cachelines: 3, members: 7 */

After:
  sgs:        /* size: 56, cachelines: 1, members: 10 */
  sds:        /* size: 152, cachelines: 3, members: 7 */

Further we can avoid clearing all of sds since we do a total
clear/assignment of sg_stats in update_sg_lb_stats() with exception of
busiest_stat.avg_load which is referenced in update_sd_pick_busiest().

Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/fair.c |   42 +++++++++++++++++++++++++++++++++++-------
 1 file changed, 35 insertions(+), 7 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4282,12 +4282,12 @@ static unsigned long task_h_load(struct
 struct sg_lb_stats {
 	unsigned long avg_load; /*Avg load across the CPUs of the group */
 	unsigned long group_load; /* Total load over the CPUs of the group */
-	unsigned long sum_nr_running; /* Nr tasks running in the group */
 	unsigned long sum_weighted_load; /* Weighted load of group's tasks */
 	unsigned long load_per_task;
-	unsigned long group_capacity;
-	unsigned long idle_cpus;
-	unsigned long group_weight;
+	unsigned int sum_nr_running; /* Nr tasks running in the group */
+	unsigned int group_capacity;
+	unsigned int idle_cpus;
+	unsigned int group_weight;
 	int group_imb; /* Is there an imbalance in the group ? */
 	int group_has_capacity; /* Is there extra capacity in the group? */
 };
@@ -4303,10 +4303,38 @@ struct sd_lb_stats {
 	unsigned long total_pwr;	/* Total power of all groups in sd */
 	unsigned long avg_load;	/* Average load across all groups in sd */
 
-	struct sg_lb_stats this_stat;	/* Statistics of this group */
 	struct sg_lb_stats busiest_stat;/* Statistics of the busiest group */
+	struct sg_lb_stats this_stat;	/* Statistics of this group */
 };
 
+static inline void init_sd_lb_stats(struct sd_lb_stats *sds)
+{
+	/*
+	 * struct sd_lb_stats {
+	 *	   struct sched_group *       busiest;             //     0  8
+	 *	   struct sched_group *       this;                //     8  8
+	 *	   long unsigned int          total_load;          //    16  8
+	 *	   long unsigned int          total_pwr;           //    24  8
+	 *	   long unsigned int          avg_load;            //    32  8
+	 *	   struct sg_lb_stats {
+	 *		   long unsigned int  avg_load;            //    40  8
+	 *		   long unsigned int  group_load;          //    48  8
+	 *	           ...
+	 *	   } busiest_stat;                                 //    40 56
+	 *	   struct sg_lb_stats	      this_stat;	   //    96 56
+	 *
+	 *	   // size: 152, cachelines: 3, members: 7
+	 *	   // last cacheline: 24 bytes
+	 * };
+	 *
+	 * Skimp on the clearing to avoid duplicate work. We can avoid clearing
+	 * this_stat because update_sg_lb_stats() does a full clear/assignment.
+	 * We must however clear busiest_stat::avg_load because
+	 * update_sd_pick_busiest() reads this before assignment.
+	 */
+	memset(sds, 0, offsetof(struct sd_lb_stats, busiest_stat.group_load));
+}
+
 /**
  * get_sd_load_idx - Obtain the load index for a given sched domain.
  * @sd: The sched_domain whose load_idx is to be obtained.
@@ -4665,7 +4693,7 @@ static inline void update_sd_lb_stats(st
 		 */
 		if (prefer_sibling && !local_group &&
 				sds->this && sds->this_stat.group_has_capacity)
-			sgs->group_capacity = min(sgs->group_capacity, 1UL);
+			sgs->group_capacity = min(sgs->group_capacity, 1U);
 
 		/* Now, start updating sd_lb_stats */
 		sds->total_load += sgs->group_load;
@@ -4896,7 +4924,7 @@ static struct sched_group *find_busiest_
 	struct sg_lb_stats *this, *busiest;
 	struct sd_lb_stats sds;
 
-	memset(&sds, 0, sizeof(sds));
+	init_sd_lb_stats(&sds);
 
 	/*
 	 * Compute the various statistics relavent for load balancing at


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