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Subject: Re: [PATCH 04/32] nohz: Separate idle sleeping time accounting from
 nohz switching
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 19:44:01 +0200
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On Mon, 2011-08-29 at 18:32 +0200, Frederic Weisbecker wrote:
> On Mon, Aug 29, 2011 at 04:23:19PM +0200, Peter Zijlstra wrote:
> > On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
> > > To prepare for having nohz mode switching independant from idle,
> > > pull the idle sleeping time accounting out of the tick stop API.
> > > 
> > > This implies to implement some new API to call when we
> > > enter/exit idle. 
> > 
> > I mean, I really love brevity, but you seem to just not state all the
> > important bits ;-)
> > 
> > So the goal is to disable the tick more often (say when running 1
> > userbound task), why does that need new hooks? If we already had the
> > tick disabled, the tick_nohz_stop_sched_tick() call on going idle will
> > simply not do anything.
> > 
> > If we go from idle to running something we want to enable the tick
> > initially because doing the task wakeup involves RCU etc.. Once we find
> > the task is indeed userbound and we've finished all our state we can
> > disable the thing again.
> 
> That's because we are going to have two different sources of stop/restarting
> the tick: either idle or a random task. In the case of idle we have very
> specific things to handle like idle time accounting, idle stats, rcu, ...
> 
> I could do these things conditionally using a some idle_cpu() checks but
> the end result would not be very proper.

Right, but you didn't explain any of that in the changelog. So the
reasoning is that because tick_nohz_stop_sched_tick() does more than
just stop the tick, and this extra work needs to be isolated to just the
idle case, therefore we need hooks specific for the idle loop.