RE: [PATCH v2] cputime: fix invalid gtime

[Hiroshi Shimamoto]

> Subject: Re: [PATCH v2] cputime: fix invalid gtime
> 
> 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.

Do you mean that task_cputime() and task_cputime_scaled() don't hit invalid behavior
but have some extra operation cost which could be removed?

Will look into it, and send patches with stable tag.

thanks,
Hiroshi

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