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

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Message-ID: <20140409112950.7be9f8ac@gandalf.local.home>
Date:	Wed, 9 Apr 2014 11:29:50 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Viresh Kumar <viresh.kumar@...aro.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
Subject: Re: nohz problem with idle time on old hardware

On Wed, 9 Apr 2014 20:50:59 +0530
Viresh Kumar <viresh.kumar@...aro.org> wrote:

> On 9 April 2014 20:01, Steven Rostedt <rostedt@...dmis.org> wrote:
> > Ouch! You are correct, this part of the patch makes no sense. That's
> > what I get for reviewing a patch and not looking at all the code around
> > the changes. (another kernel developer hangs head in shame :-( )
> >
> > I think that if statement should be nuked.
> 
> Hmm, my opinion differs here :)
> 
> If we completely remove this statement, we will run
> tick_nohz_switch_to_nohz() even if nohz is not enabled. And check for
> enabled must stay.

Do we? This is only called by tick_check_oneshot_change() which has the
following:

int tick_check_oneshot_change(int allow_nohz)
{
	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);

	if (!test_and_clear_bit(0, &ts->check_clocks))
		return 0;

	if (ts->nohz_mode != NOHZ_MODE_INACTIVE)
		return 0;

	if (!timekeeping_valid_for_hres() || !tick_is_oneshot_available())
		return 0;

	if (!allow_nohz)
		return 1;

	tick_nohz_switch_to_nohz();
	return 0;
}

How often does it make it to that last check?

-- Steve
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