[PATCH v3 1/2] cputime: fix invalid gtime in proc

[Hiroshi Shimamoto]

From: Hiroshi Shimamoto <h-shimamoto@xxxxxxxxxxxxx>

/proc/stats shows invalid gtime when the thread is running in guest.
When vtime accounting is not enabled, we cannot get a valid delta.
The delta is calculated now - tsk->vtime_snap, but tsk->vtime_snap
is only updated when vtime accounting is enabled.

This patch makes task_gtime() just return gtime when vtime accounting
is not enabled.

Use context_tracking_is_enabled() to check if the vtime accounting on
current cpu. In future we should check the state of cpu which is running
target thread.

The kernel config contains CONFIG_VIRT_CPU_ACCOUNTING_GEN=y and
CONFIG_NO_HZ_FULL_ALL=n and boot without nohz_full.

I ran and stop a busy loop in VM and see the gtime in host.
Dump the 43rd field which shows the gtime in every second.
 # while :; do awk '{print $3" "$43}' /proc/3955/task/4014/stat; sleep 1; done
S 4348
R 7064566
R 7064766
R 7064967
R 7065168
S 4759
S 4759

During running busy loop, it returns large value.

After applying this patch, we can see right gtime.

 # while :; do awk '{print $3" "$43}' /proc/10913/task/10956/stat; sleep 1; done
S 5338
R 5365
R 5465
R 5566
R 5666
S 5726
S 5726

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
v2: Update ChangeLog to put the script and show the point.
v3: Use context_tracking_is_enabled() instead of vtime_accounting_enabled().

 kernel/sched/cputime.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 8cbc3db..63904e7 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -786,6 +786,9 @@ cputime_t task_gtime(struct task_struct *t)
 	unsigned int seq;
 	cputime_t gtime;
 
+	if (!context_tracking_is_enabled())
+		return t->gtime;
+
 	do {
 		seq = read_seqbegin(&t->vtime_seqlock);
 
-- 
1.8.3.1

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