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SubjectRe: [PATCH 09/32] nohz: Move ts->idle_calls into strict idle logic
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On Mon, 2011-08-29 at 19:34 +0200, Frederic Weisbecker wrote:
> 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.

Then call it tick_nohz_can_stop_tick_idle() or so, and create
tick_nohz_can_stop_tick() to deal with all stuff.

> 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.

Well, no, on interrupt return you shouldn't do anything. If you've
stopped the tick it stays stopped until you do something that needs it,
then that action will re-enable it.

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

I would expect the tick not to be stopped when tick_nohz_can_stop_tick()
returns false. If it returns true, then I expect anything that needs it
to re-enable it.



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