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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 13:19:18 +0200
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On Tue, 2011-08-30 at 01:35 +0200, Frederic Weisbecker wrote:
> 
> OTOH it is needed to find non-critical sections when asked to cooperate
> in a grace period completion. But if no callback have been enqueued on
> the whole system we are fine. 

Its that 'whole system' clause that I have a problem with. It would be
perfectly fine to have a number of cpus very busy generating rcu
callbacks, however this should not mean our adaptive nohz cpu should be
bothered to complete grace periods.

Requiring it to participate in the grace period state machine is a fail,
plain and simple.