Date: Wed, 4 Nov 2015 17:14:57 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>,
        Ingo Molnar <mingo@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v3 1/2] cputime: fix invalid gtime in proc
Message-ID: <20151104161455.GB12889@lerouge>
References: <7F861DC0615E0C47A872E6F3C5FCDDBD05F575AA@BPXM14GP.gisp.nec.co.jp>
 <20151102161351.GT3604@twins.programming.kicks-ass.net>
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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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