linux-kernel - Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

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Date:   Thu, 20 Jul 2017 16:19:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Aubrey Li <aubrey.li@...el.com>, len.brown@...el.com,
        rjw@...ysocki.net, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, yang.zhang.wz@...il.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, daniel.lezcano@...aro.org
Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

On Thu, Jul 20, 2017 at 05:50:54AM -0700, Paul E. McKenney wrote:
> > 
> > static void cpuidle_idle_call()
> > {
> > 	rcu_idle_enter()
> > 	......
> > 	rcu_idle_exit()
> > }
> > 
> > I want
> > 
> > static void cpuidle_idle_call()
> > {
> > 	if (tick stopped)
> > 		rcu_idle_enter()
> > 	......
> > 	if (tick stopped)
> > 		rcu_idle_exit()
> > }
> >
> > Or checking tick stop can be put into rcu_idle_enter/exit
> 
> The answer is the traditional "it depends".
> 
> If the above change was all that you did, that would be a bug in the
> case where the predicted short idle time turned out to in reality be an
> indefinite idle time. 

Can't be, you didn't disable the tick after all, so you're guaranteed to
get interrupted by the tick and try again.