Re: [PATCH 09/32] nohz: Move ts->idle_calls into strict idle logic

Frederic Weisbecker
2011-08-29T17:34:39Z

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...