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

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Message-ID: <20151104161455.GB12889@lerouge>
Date:	Wed, 4 Nov 2015 17:14:57 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/2] cputime: fix invalid gtime in proc

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
> > 
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