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Subject: [PATCH] tg: show the sum wait time of an task group
From:   =?UTF-8?B?546L6LSH?= <yun.wang@linux.alibaba.com>
To:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org
References: <5c4c978d-e8fb-4bcb-b942-3c6d3dcfc13e@linux.alibaba.com>
Message-ID: <b9e74571-87d0-ef27-3f14-ebd21d72ee15@linux.alibaba.com>
Date:   Tue, 3 Jul 2018 13:42:20 +0800
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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 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@linux.alibaba.com>
---
Since RFC:
   redesigned the way to acquire wait_sum

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

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 78d8fac..cbff06b 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 wait_sum = 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()) {
+		for_each_possible_cpu(i)
+			wait_sum += tg->se[i]->statistics.wait_sum;
+		seq_printf(sf, "wait_sum %llu\n", wait_sum);
+	}
+
  	return 0;
  }
  #endif /* CONFIG_CFS_BANDWIDTH */
-- 
1.8.3.1