linux-kernel - Re: [Query]: tick-sched: why don't we stop tick when we are running idle task?

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Message-ID: <CAKohpom5Ft_XrNTExE9Ukyv1e4car+wicDiPp_Bdhy=PW9Sq8g@mail.gmail.com>
Date:	Wed, 9 Apr 2014 16:19:44 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>
Subject: Re: [Query]: tick-sched: why don't we stop tick when we are running
 idle task?

On 9 April 2014 16:03, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> Hi Frederic,
>
> File: kernel/time/tick-sched.c
> Function: tick_nohz_full_stop_tick()
>
> We are doing this:
>
> if (!tick_nohz_full_cpu(cpu) || is_idle_task(current))
>         return;
>
> Which means: if a FULL_NO_HZ cpu is running idle task currently,
> don't stop its tick..
>
> I couldn't understand why. Can you please help here?

Is it because of this code ? :

void tick_nohz_irq_exit(void)
{
        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);

        if (ts->inidle)
                __tick_nohz_idle_enter(ts);
        else
                tick_nohz_full_stop_tick(ts);
}

i.e. tick_nohz_full_stop_tick() would never be called while we have
'inidle' set or we are running idle task?

In this case as well, we don't really need that check to be present.
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