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Subject: Re: [PATCH 14/32] nohz/cpuset: Don't turn off the tick if rcu needs
 it
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>,
        Ingo Molnar <mingo@elte.hu>, Lai Jiangshan <laijs@cn.fujitsu.com>,
        "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: Mon, 29 Aug 2011 17:36:31 +0200
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On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
> @@ -2446,6 +2447,19 @@ bool cpuset_nohz_can_stop_tick(void)
>         if (rq->nr_running > 1)
>                 return false;
>  
> +       cpu = smp_processor_id();
> +
> +       /*
> +        * FIXME: will probably be removed soon as it's
> +        * already checked from tick_nohz_stop_sched_tick()
> +        */
> +       if (rcu_needs_cpu(cpu))
> +               return false;
> +
> +       /* Is there a grace period to complete ? */
> +       if (rcu_pending(cpu))
> +               return false;
> +
>         return true;
>  } 

This really shouldn't live in sched.c, also I would have expected
tick_nohz_can_stop_tick() to do this, its named like it does this.