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SubjectRe: [PATCH RFC tip/core/rcu 1/3] rcu: The Bloatwatch Edition, v7
Steven Rostedt wrote:
> But isn't the tick_nohz_stop_sched_tick have several exits?
>
> void tick_nohz_stop_sched_tick(int inidle)
> {
> [..]
>
> if (!inidle && !ts->inidle)
> goto end;
>
> ts->inidle = 1;
>
> [..]
>
> if (!ts->tick_stopped) {
> [..]
> ts->tick_stopped = 1;
> ts->idle_jiffies = last_jiffies;
> rcu_enter_nohz();
> }
> [..]
>
>
> So I'm not sure calling tick_nohz_stop_sched_tick twice equals calling
> rcu_enter_nohz twice.
>

Hi, tglx, steven,

(Thank to tglx for helping me at the Japan Linux Symposium)

I found something weird about NO_HZ, maybe I misunderstood the codes.

see this flow:

cpu idle
enter nohz
cpu halt
---->interrupt happens
irq_enter()
we don't reprogram the clock device #1
irq_exit()
tick_nohz_stop_sched_tick(inidle = 0)
something disallow this cpu reenter nohz #2
we don't reprogram the clock device #3
<----interrupt return
cpu halt again and wait interrupt for a long time than expected #4
exit nohz

#1 tick_nohz_kick_tick() is disabled in the current mainline kernel,
so we don't calls tick_nohz_restart(ts, now) when irq_enter()

static void tick_nohz_kick_tick(int cpu)
{
#if 0 <------------- here
/* Switch back to 2.6.27 behaviour */
struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
ktime_t delta, now;

if (!ts->tick_stopped)
return;
/*
* Do not touch the tick device, when the next expiry is either
* already reached or less/equal than the tick period.
*/
now = ktime_get();
delta = ktime_sub(hrtimer_get_expires(&ts->sched_timer), now);
if (delta.tv64 <= tick_period.tv64)
return;
tick_nohz_restart(ts, now); <----------- here
#endif
}

#2 When rcu_needs_cpu() or printk_needs_cpu()
returns true then tick_nohz_stop_sched_tick() will just return.

#3 And we don't reprogram the clock device when #2 happens

#4 So we may be in nohz for a long time than expected, but actually
we have some work to do. (rcu, printk... etc)

So I think, we need to reprogram the clock device and restart the tick
when #2 happens, or there is something that I have misunderstood.

Thanks, Lai



> -- Steve
>
>>> So I do believe that rcu_enter_nohz() and rcu_exit_nohz() are in fact
>>> invoked in pairs. One strange thing about this is that the idle loop
>>> first invokes rcu_enter_nohz(), then invokes rcu_exit_nohz(), while
>>> an interrupt handler first invokes rcu_irq_enter() and then invokes
>>> rcu_irq_exit(). So the idle loop enters dyntick-idle mode and then
>>> leaves it, while an interrupt handler might leave dyntick-idle mode and
>>> then re-enter it.
>>>
>>> Or am I still missing something here?
>>>
>>> Thanx, Paul
>>>
>>>
>
>
>




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