If a sched group has passed the test for sufficient load in
update_sg_lb_stats,to qualify for load balancing,then PJT's
metrics has to be used to qualify the right sched group as the busiest group.
The scenario which led to this patch is shown below:
Consider Task1 and Task2 to be a long running task
and Tasks 3,4,5,6 to be short running tasks
Task3
Task4
Task1 Task5
Task2 Task6
------ ------
SCHED_GRP1 SCHED_GRP2
Normal load calculator would qualify SCHED_GRP2 as
the candidate for sd->busiest due to the following loads
that it calculates.
SCHED_GRP1:2048
SCHED_GRP2:4096
Load calculator would probably qualify SCHED_GRP1 as the candidate
for sd->busiest due to the following loads that it calculates
SCHED_GRP1:3200
SCHED_GRP2:1156
This patch aims to strike a balance between the loads of the
group and the number of tasks running on the group to decide the
busiest group in the sched_domain.
This means we will need to use the PJT's metrics but with an
additional constraint.
Signed-off-by: Preeti U Murthy <pre...@linux.vnet.ibm.com>
---
kernel/sched/fair.c | 25 ++++++++++++++++++++++---
1 file changed, 22 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e02dad4..aafa3c1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -165,7 +165,8 @@ void sched_init_granularity(void)
#else
# define WMULT_CONST (1UL << 32)
#endif
-
+#define NR_THRESHOLD 2
+#define LOAD_THRESHOLD 1
#define WMULT_SHIFT 32
/*
@@ -4169,6 +4170,7 @@ struct sd_lb_stats {
/* Statistics of the busiest group */
unsigned int busiest_idle_cpus;
unsigned long max_load;
+ u64 max_sg_load; /* Equivalent of max_load but calculated using pjt's
metric*/
unsigned long busiest_load_per_task;
unsigned long busiest_nr_running;
unsigned long busiest_group_capacity;
@@ -4628,8 +4630,24 @@ static bool update_sd_pick_busiest(struct lb_env *env,
struct sched_group *sg,
struct sg_lb_stats *sgs)
{
- if (sgs->avg_load <= sds->max_load)
- return false;
+ /* Use PJT's metrics to qualify a sched_group as busy
+ *
+ * But a low load sched group may be queueing up many tasks
+ * So before dismissing a sched group with lesser load,ensure
+ * that the number of processes on it is checked if it is
+ * not too less loaded than the max load so far
+ *
+ * But as of now as LOAD_THRESHOLD is 1,this check is a nop.
+ * But we could vary LOAD_THRESHOLD suitably to bring in this check
+ */
+ if (sgs->avg_cfs_runnable_load <= sds->max_sg_load) {
+ if (sgs->avg_cfs_runnable_load > LOAD_THRESHOLD *
sds->max_sg_load) {
+ if (sgs->sum_nr_running <= (NR_THRESHOLD +
sds->busiest_nr_running))
+ return false;
+ } else {
+ return false;
+ }
+ }
if (sgs->sum_nr_running > sgs->group_capacity)
return true;
@@ -4708,6 +4726,7 @@ static inline void update_sd_lb_stats(struct lb_env *env,
sds->this_idle_cpus = sgs.idle_cpus;
} else if (update_sd_pick_busiest(env, sds, sg, &sgs)) {
sds->max_load = sgs.avg_load;
+ sds->max_sg_load = sgs.avg_cfs_runnable_load;
sds->busiest = sg;
sds->busiest_nr_running = sgs.sum_nr_running;
sds->busiest_idle_cpus = sgs.idle_cpus;
--
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