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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Feb 03, 2014 at 12:21:16PM +0530, Viresh Kumar wrote:
> Sorry was away for short vacation.
> 
> On 28 January 2014 19:20, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > On Thu, Jan 23, 2014 at 07:50:40PM +0530, Viresh Kumar wrote:
> >> 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?
> 
> Something is seriously wrong with me, again wrote rubbish code.
> Let me phrase what I wanted to write :)
> 
> "don't send IPI to a idle CPU for a deferrable timer."
> 
> Probably I code it correctly this time atleast.
> 
> -       wake_up_nohz_cpu(cpu);
> +       if (!(tbase_get_deferrable(timer->base) && idle_cpu(cpu)))
> +               wake_up_nohz_cpu(cpu);

Yeah but that's racy if the target is nohz full. We may be seeing it idle whereas
it woke up lately and run in userspace tickless for a while.

> 
> > 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?
> 
> Yeah, sorry for doing it for second time :(

I'm certainly not blaming you for being confused, that would be the pot calling the kettle black ;)

> 
> > 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).
> 
> I wasn't looking at this problem with NO_HZ_FULL in mind. As I thought its
> only about if the CPU is idle or not. And so the solution I was
> talking about was:
> 
> "don't send IPI to a idle CPU for a deferrable timer."
> 
> But I see that still failing with the code you wrote. For normal cases where we
> don't enable NO_HZ_FULL, we will still end up waking up idle CPUs which
> is what Lei Wen reported initially.

Not with the small change I proposed above.
I'm applying it.

> 
> Also if a CPU is marked for NO_HZ_FULL and is not idle currently then we
> wouldn't send a IPI for a deferrable timer. But we actually need that, so that
> we can reevaluate the timers order again?

Right.