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

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Message-ID: <20151113153300.GB14397@lerouge>
Date:	Fri, 13 Nov 2015 16:33:01 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 Tue, Nov 10, 2015 at 08:43:17AM +0000, Hiroshi Shimamoto wrote:
> > 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")
> 
> Is this patch going to apply to fix the regression?

Peter, if you have doubts, I can integrate this change in a larger
series that does a proper cleanup. But this very patch will still need
to be tagged as stable for backport.

Thanks.
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