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Subject: Re: [PATCH 13/32] nohz: Adaptive tick stop and restart on nohz
 cpuset
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 20:07:10 +0200
In-Reply-To: <20110829180222.GG9748@somewhere.redhat.com>
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On Mon, 2011-08-29 at 20:02 +0200, Frederic Weisbecker wrote:
> On Mon, Aug 29, 2011 at 05:28:09PM +0200, Peter Zijlstra wrote:
> > On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
> > > +bool cpuset_nohz_can_stop_tick(void)
> > > +{
> > > +       struct rq *rq;
> > > +
> > > +       rq = this_rq();
> > > +
> > > +       /* More than one running task need preemption */
> > > +       if (rq->nr_running > 1)
> > > +               return false;
> > > +
> > > +       return true;
> > > +} 
> > 
> > int sched_needs_cpu(int cpu), seems the right name, matches the existing
> > {rcu,printk,arch}_needs_cpu() functions.
> 
> tick_nohz_stop_sched_tick() already handles that by keeping a periodic
> behaviour if one of these conditions are met.

What? tick_nohz_stop_sched_tick() most surely cannot access struct rq,
so it cannot do the nr_running test.

> It has also the upside to restore the periodic behaviour if needed
> from irq return if the tick was stopped.

Again, what?