[RFD 5/9] per-cgroup boot time

[Glauber Costa <glommer@parallels.com>]

Record the time in which the cgroup was created. This can be
used to provide a more accurate boottime information in
cpuacct.proc.stat.

Signed-off-by: Glauber Costa <glommer@parallels.com>
---
 kernel/sched.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 9c6e44e..7612410 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -280,6 +280,7 @@ struct task_group {
 #endif
 	unsigned long total_forks;
 	struct kernel_stat __percpu *cpustat;
+	struct timespec start_time;
 };
 
 /* task_group_lock serializes the addition/removal of task groups */
@@ -8141,6 +8142,7 @@ void __init sched_init(void)
 	INIT_LIST_HEAD(&root_task_group.siblings);
 	autogroup_init(&init_task);
 
+	root_task_group.start_time = (struct timespec){0, 0};
 	root_task_group.cpustat = alloc_percpu(struct kernel_stat);
 	/* Failing that early an allocation means we're screwed anyway */
 	BUG_ON(!root_task_group.cpustat);
@@ -8619,6 +8621,8 @@ struct task_group *sched_create_group(struct task_group *parent)
 		kstat->cpustat[IOWAIT_BASE] = root_kstat->cpustat[IOWAIT];
 	}
 
+	get_monotonic_boottime(&tg->start_time);
+
 	spin_lock_irqsave(&task_group_lock, flags);
 	list_add_rcu(&tg->list, &task_groups);
 
@@ -9154,6 +9158,7 @@ int cpu_cgroup_proc_stat(struct cgroup *cgrp, struct cftype *cft, struct seq_fil
 		irq = softirq = steal = cputime64_zero;
 	guest = guest_nice = cputime64_zero;
 	getboottime(&boottime);
+	boottime = timespec_add(boottime, tg->start_time);
 	jif = boottime.tv_sec;
 
 	for_each_possible_cpu(i) {
-- 
1.7.6