linux-kernel - Re: [BUG nohz]: wrong user and system time accounting

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Date:   Tue, 28 Mar 2017 17:30:05 -0400
From:   Luiz Capitulino <lcapitulino@...hat.com>
To:     Rik van Riel <riel@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [BUG nohz]: wrong user and system time accounting

On Tue, 28 Mar 2017 17:24:11 -0400
Rik van Riel <riel@...hat.com> wrote:

> On Tue, 2017-03-28 at 16:14 -0400, Luiz Capitulino wrote:
> 
> > And I think I was right, it looks like the nohz code is programming
> > the tick period incorrectly when restarting the tick. The patch below
> > fixes things for me, but I still have some homework todo and more
> > testing before posting a patch for inclusion. Could you guys test it?  
> 
> I spoke too soon.  After half an hour of runtime,
> things have gotten aligned to give me about 50/50
> user time and system time with your test case,
> again.

Hmmm, maybe it's incomplete. I still think that nohz might screwing
something up when re-activating the tick.

> 
> This is on an 8 VCPU virtual machine, with
> nohz_full=2-7, and the test case running on one
> of the nohz_full CPUs.
> 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 7fe53be..9abe979 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -1152,6 +1152,7 @@ static enum hrtimer_restart
> > tick_sched_timer(struct hrtimer *timer)
> >         struct pt_regs *regs = get_irq_regs();
> >         ktime_t now = ktime_get();
> >  
> > +       ts->last_tick = now;
> >         tick_sched_do_timer(now);
> >  
> >         /*  
>