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

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Date:   Tue, 28 Mar 2017 17:24:11 -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:

> 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.

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);
>  
>         /*