linux-kernel - Re: [PATCH 09/10] nohz: Switch nohz full timekeeper to dynticks idle on top of sysidle detection

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Message-ID: <20140729220443.GD14520@localhost.localdomain>
Date:	Wed, 30 Jul 2014 00:04:45 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 09/10] nohz: Switch nohz full timekeeper to dynticks idle
 on top of sysidle detection

On Tue, Jul 29, 2014 at 02:17:53PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 28, 2014 at 07:37:35PM +0200, Frederic Weisbecker wrote:
> 
> > @@ -745,7 +747,7 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
> >  		 * if there are full dynticks CPUs around
> >  		 */
> >  		if (tick_do_timer_cpu == cpu)
> > -			return false;
> > +			return rcu_sys_is_idle();
> >  	}
> 
> I still feel its entirely upside down to rely on RCU for this...

RCU had this feature internally for a while. Then comes nohz. I guess in
a perfect world we would make it a library that RCU and nohz could use.
It's probably possible to do but right now it is too tied to RCU internals.
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