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

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Message-ID: <20131113111257.482c2955@gandalf.local.home>
Date:	Wed, 13 Nov 2013 11:12:57 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Matthew Whitehead <tedheadster@...il.com>, john.stultz@...aro.org,
	linux-kernel@...r.kernel.org, mwhitehe@...hat.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: nohz problem with idle time on old hardware

On Wed, 13 Nov 2013 17:07:18 +0100 (CET)
Thomas Gleixner <tglx@...utronix.de> wrote:


> Right. It's telling you if NOHZ is enabled. It's not telling you that
> NOHZ is active.

Yeah, which makes this code rather silly:

in rcu_prepare_for_idle():

	/* Handle nohz enablement switches conservatively. */
	tne = ACCESS_ONCE(tick_nohz_enabled);
	if (tne != rdtp->tick_nohz_enabled_snap) {
		if (rcu_cpu_has_callbacks(cpu, NULL))
			invoke_rcu_core(); /* force nohz to see update. */
		rdtp->tick_nohz_enabled_snap = tne;
		return;
	}

-- Steve
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