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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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