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

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Message-ID: <7F861DC0615E0C47A872E6F3C5FCDDBD05F6B430@BPXM14GP.gisp.nec.co.jp>
Date:	Tue, 10 Nov 2015 08:43:17 +0000
From:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
To:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	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

> 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?

thanks,
Hiroshi

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