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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Nov 02, 2015 at 05:13:51PM +0100, Peter Zijlstra wrote:
> On Fri, Oct 30, 2015 at 12:46:39AM +0000, Hiroshi Shimamoto wrote:
> > +++ 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;
> > +
> 
> Yeah, not happy about that.. why do we have to touch context tracking
> muck to find vtime state etc.

That's right, this is because it is deemed to be a quick and non invasive fix
to be backported.

Then will come the more invasive but proper fix consisting in having
vtime_accounting_enabled() telling if vtime is running on any CPU and
vtime_accounting_cpu_enabled(). The first will be used for remote readers
(as in this patch) and the second for writers.

Since we are dealing with a regression, it's better to minimize the changes.
AFAICT, the regression got introduced in 2012:

  6a61671bb2f3a1bd12cd17b8fca811a624782632
  ("cputime: Safely read cputime of full dynticks CPUs")

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