[RFC PATCH] tg: count the sum wait time of an task group

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Although we can rely on cpuacct to present the cpu usage of task
group, it is hard to tell how intense the competition is between
these groups on cpu resources.

Monitoring the wait time of each process could cost too much, and
there is no good way to accurately represent the conflict with
these info, we need the wait time on group dimension.

Thus we introduced group's wait_sum provided by kernel to represent
the conflict between task groups, whenever a group's cfs_rq ends
waiting, it's wait time accounted to the sum.

The cpu.stat is modified to show the new statistic, like:

  nr_periods 0
  nr_throttled 0
  throttled_time 0
  wait_sum 2035098795584

Now we can monitor the changing on wait_sum to tell how suffering
a task group is in the fight of cpu resources.

Signed-off-by: Michael Wang <yun.wang@xxxxxxxxxxxxxxxxx>
---
 kernel/sched/core.c  | 2 ++
 kernel/sched/fair.c  | 4 ++++
 kernel/sched/sched.h | 1 +
 3 files changed, 7 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 78d8fac..ac27b8d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6787,6 +6787,8 @@ static int cpu_cfs_stat_show(struct seq_file *sf, 
void *v)
 	seq_printf(sf, "nr_periods %d\n", cfs_b->nr_periods);
 	seq_printf(sf, "nr_throttled %d\n", cfs_b->nr_throttled);
 	seq_printf(sf, "throttled_time %llu\n", cfs_b->throttled_time);
+	if (schedstat_enabled())
+		seq_printf(sf, "wait_sum %llu\n", tg->wait_sum);

 	return 0;
 }
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1866e64..ef82ceb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -862,6 +862,7 @@ static void update_curr_fair(struct rq *rq)
 static inline void
 update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
+	struct task_group *tg;
 	struct task_struct *p;
 	u64 delta;

@@ -882,6 +883,9 @@ static void update_curr_fair(struct rq *rq)
 			return;
 		}
 		trace_sched_stat_wait(p, delta);
+	} else {
+		tg = group_cfs_rq(se)->tg;
+		__schedstat_add(tg->wait_sum, delta);
 	}

 	__schedstat_set(se->statistics.wait_max,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 6601baf..bb9b4fb 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -358,6 +358,7 @@ struct task_group {
 	/* runqueue "owned" by this group on each CPU */
 	struct cfs_rq		**cfs_rq;
 	unsigned long		shares;
+	u64			wait_sum;

 #ifdef	CONFIG_SMP
 	/*
--
1.8.3.1