linux-kernel - Re: nohz problem with idle time on old hardware

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Message-ID: <CAKohpok8e9MoWFeNquMreRR8dbBP-_9CDP7=xdOwAwWv87VZAQ@mail.gmail.com>
Date:	Wed, 9 Apr 2014 21:26:51 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Matthew Whitehead <tedheadster@...il.com>,
	John Stultz <john.stultz@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	mwhitehe@...hat.com, Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: nohz problem with idle time on old hardware

On 9 April 2014 21:09, Steven Rostedt <rostedt@...dmis.org> wrote:
> Reading even more of the code, now I'm totally confused :-)

:)

> When tick_setup_sched_timer() is called, if tick_nohz_enabled is set,
> then we set tick_nohz_active.

correct.

> This gets called by hrtimer_switch_to_hres(), and before that is
> called, the tick_check_oneshot_changed() will never get to the
> tick_nohz_switch_to_nohz() call.

If hrtimer_switch_to_hres() is called or HRES is enabled, we will
never ever call tick_nohz_switch_to_nohz().

> Looks to me, the real answer is to nuke both the if statement *and* the
> setting of the tick_nohz_active in that function. Both looks a bit
> redundant to me.

When HRES isn't enabled and NOHZ isn't enabled as well, in that
case we should stick to the periodic code from tick-common.c and
the oneshot options of tick_nohz_switch_to_nohz() or
hrtimer_switch_to_hres() shouldn't be used. And so, we still need
those checks, as per my understanding. :)

Lets see what others have for this discussion :)
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