linux-kernel - [PATCH v2] tg: show the sum wait time of an task group

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Date:   Wed, 4 Jul 2018 11:27:27 +0800
From:   王贇 <yun.wang@...ux.alibaba.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2] tg: show the sum wait time of an task group

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 or sched_debug 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 represent the conflict between
task groups, which is simply sum the wait time of group's cfs_rq.

The 'cpu.stat' is modified to show the 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.

For example:
   (wait_sum - last_wait_sum) * 100 / (nr_cpu * period_ns) == X%

means the task group paid X percentage of period on waiting
for the cpu.

Signed-off-by: Michael Wang <yun.wang@...ux.alibaba.com>
---

Since v1:
   Use schedstat_val to avoid compile error
   Check and skip root_task_group

  kernel/sched/core.c | 8 ++++++++
  1 file changed, 8 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 78d8fac..80ab995 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6781,6 +6781,8 @@ static int __cfs_schedulable(struct task_group *tg, u64 period, u64 quota)

  static int cpu_cfs_stat_show(struct seq_file *sf, void *v)
  {
+	int i;
+	u64 ws = 0;
  	struct task_group *tg = css_tg(seq_css(sf));
  	struct cfs_bandwidth *cfs_b = &tg->cfs_bandwidth;

@@ -6788,6 +6790,12 @@ static int cpu_cfs_stat_show(struct seq_file *sf, void *v)
  	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() && tg != &root_task_group) {
+		for_each_possible_cpu(i)
+			ws += schedstat_val(tg->se[i]->statistics.wait_sum);
+		seq_printf(sf, "wait_sum %llu\n", ws);
+	}
+
  	return 0;
  }
  #endif /* CONFIG_CFS_BANDWIDTH */
-- 
1.8.3.1