Re: [PATCH v2] cputime: fix invalid gtime

[Frederic Weisbecker]

On Wed, Oct 28, 2015 at 07:01:18AM +0000, Hiroshi Shimamoto wrote:
> 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.
> 
> 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>
> ---
>  kernel/sched/cputime.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 8cbc3db..f614ee9 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 (!vtime_accounting_enabled())
> +		return t->gtime;
> +

Obviously I completely messed up there. And task_cputime() has a similar issue
but it happens to work due to vtime_snap_whence set to VTIME_SLEEPING when vtime
doesn't run. Still it works at the cost of a seqcount read operation.

Do you think you could fix it too (along with task_cputime_scaled())? I think those
patches will also need a stable tag.

Thanks!



>  	do {
>  		seq = read_seqbegin(&t->vtime_seqlock);
>  
> -- 
> 1.8.3.1
> 
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