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Subject: Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to
 idle enter/exit APIs
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anton Blanchard <anton@au1.ibm.com>, Avi Kivity <avi@redhat.com>,
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        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Paul Menage <menage@google.com>,
        Stephen Hemminger <shemminger@vyatta.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Pepper <lnxninja@linux.vnet.ibm.com>
Date: Tue, 30 Aug 2011 13:21:55 +0200
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On Tue, 2011-08-30 at 01:35 +0200, Frederic Weisbecker wrote:
> > That means it has to be in an extended grace period when we stop the
> > tick.
> 
> You mean extended quiescent state?

Yeah that :-)

> As a summary here is what we do:
> 
> - if we are in the kernel, we can't run into extended quiescent state because
> we may make use of rcu anytime there. But if we run nohz we don't have the tick
> to notice quiescent states to the RCU machinery and help completing grace periods
> so as soon as we receive an rcu IPI from another CPU (due to the grace period
> beeing extended because our nohz CPU doesn't report quiescent states), we restart
> the tick. We are optimistic enough to consider that we may avoid a lot of ticks
> even if there are some risks to be disturbed in some random rates.
> So even with the IPI we consider it as an upside.
> 
> - if we are in userspace we can run in extended quiescent state.

But you can only disable the tick/enter extended quiescent state while
in kernel-space. Thus the second clause is precluded from ever being
true.