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

[Frederic Weisbecker]

On Thu, Jan 23, 2014 at 07:50:40PM +0530, Viresh Kumar wrote:
> 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?

Well, this is going to wake up the target from its idle state, which is
what we want to avoid if the timer is deferrable, right?

The simplest thing we want is:

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

This spares the IPI for the common case where the timer is deferrable and we run
in periodic or dynticks-idle mode (which should be 99.99% of the existing workloads).

Then we can later optimize that and spare the IPI on full dynticks CPUs when they run
idle, but that require some special care about subtle races which can't be dealt
with a simple test on "idle_cpu(target)". And power consumption in full dynticks
is already very suboptimized anyway.

So I suggest we start simple with the above test, and a big fat comment which explains
what we are doing and what needs to be done in the future.

Thanks!
	

> 
> > 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.

Sure that's fine, I'll fix that along aside your change.
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