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

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Date:   Tue, 28 Mar 2017 17:01:52 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Luiz Capitulino <lcapitulino@...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, 2017-03-28 at 16:14 -0400, Luiz Capitulino wrote:
> On Tue, 28 Mar 2017 13:24:06 -0400
> Luiz Capitulino <lcapitulino@...hat.com> wrote:
> > I'm starting to suspect that the nohz code may be programming
> > the tick period to be shorter than 1ms when it re-activates
> > the tick.
> 
> 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?

Your patch seems to work. I don't claim to understand why
your patch makes a difference, but for this particular test
case, on this particular setup, it seems to work...

> 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);
>  
>         /*