Re: Is it ok for deferrable timer wakeup the idle cpu?

From: Viresh Kumar
Date: Thu Jan 23 2014 - 09:20:47 EST

On 23 January 2014 19:05, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> On Thu, Jan 23, 2014 at 11:22:32AM +0530, Viresh Kumar wrote:
>> I think below diff might get this fixed for you, though I am not sure if it
>> breaks something else. Probably Thomas/Frederic can answer here.
>> If this looks fine I will send it formally again:
>> diff --git a/kernel/timer.c b/kernel/timer.c
>> index accfd24..3a2c7fa 100644
>> --- a/kernel/timer.c
>> +++ b/kernel/timer.c
>> @@ -940,7 +940,8 @@ void add_timer_on(struct timer_list *timer, int cpu)
>> * makes sure that a CPU on the way to stop its tick can not
>> * evaluate the timer wheel.
>> */
>> - wake_up_nohz_cpu(cpu);
>> + if (!tbase_get_deferrable(timer->base))
>> + wake_up_nohz_cpu(cpu);

Wait, I got the wrong code here. That's wasn't my initial intention.
I actually wanted to write something like this:

- wake_up_nohz_cpu(cpu);
+ if (!tbase_get_deferrable(timer->base) || idle_cpu(cpu))
+ wake_up_nohz_cpu(cpu);

Will that work?

> So you simply rely on the next tick to see the new timer. This should work with
> CONFIG_NO_HZ_IDLE but not with CONFIG_NO_HZ_FULL since the target may be running
> without the tick.
> Basically, in the case of a deferrable timer you need to manage to call
> wake_up_full_nohz_cpu() but not wake_up_idle_cpu().
> It should be even possible to spare the IPI in a full dynticks CPU if it is
> running idle. But that's an optional bonus because it require some deep
> care on complicated races against the call to tick_nohz_idle_exit().
> I also realize than when we enqueue a timer on a full nohz CPU, we should set_need_resched()
> the target before sending the IPI if it is idle like does wake_up_idle_cpu(). Otherwise the
> IPI will be ignored without exiting the idle loop nor reevaluating the tick on irq exit.
> If you can fix that along the way, that will be much appreciated.

I haven't thought much about this currently as I have limited knowledge of
these routines. Though the problem we were facing wasn't related to
NO_HZ_FULL. It was just about waking up an idle cpu.
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