linux-kernel - Re: [PATCH 09/32] nohz: Move ts->idle

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Date:	Mon, 29 Aug 2011 19:34:29 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [PATCH 09/32] nohz: Move ts->idle_calls into strict idle logic

On Mon, Aug 29, 2011 at 04:47:47PM +0200, Peter Zijlstra wrote:
> On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
> > +static bool tick_nohz_can_stop_tick(int cpu, struct tick_sched *ts)
> > +{
> > +       /*
> > +        * If this cpu is offline and it is the one which updates
> > +        * jiffies, then give up the assignment and let it be taken by
> > +        * the cpu which runs the tick timer next. If we don't drop
> > +        * this here the jiffies might be stale and do_timer() never
> > +        * invoked.
> > +        */
> > +       if (unlikely(!cpu_online(cpu))) {
> > +               if (cpu == tick_do_timer_cpu)
> > +                       tick_do_timer_cpu = TICK_DO_TIMER_NONE;
> > +       }
> > +
> > +       if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
> > +               return false;
> > +
> > +       if (need_resched())
> > +               return false;
> > +
> > +       if (unlikely(local_softirq_pending() && cpu_online(cpu))) {
> > +               static int ratelimit;
> > +
> > +               if (ratelimit < 10) {
> > +                       printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n",
> > +                              (unsigned int) local_softirq_pending());
> > +                       ratelimit++;
> > +               }
> > +               return false;
> > +       }
> > +
> > +       return true;
> > +} 
> 
> Why aren't rcu_needs_cpu(), printk_needs_cpu() and arch_needs_cpu() not
> in there?
> 
> That are typical 'can we go sleep now?' functions.

Because when one of these functions are positive, the ts->next_jiffies and
ts->last_jiffies stats are updated. Not with the above.
Also I start to think the above checks are only useful in the idle case.

We still want tick_nohz_stop_sched_tick() to have the *needs_cpu() checks
so that they can restore a HZ periodic behaviour on interrupt return if
needed.

That said I wonder if some of the above conditions should restore a periodic
behaviour on interrupt return...
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