Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to idle enter/exit APIs

Peter Zijlstra
2011-08-30T11:22:13Z

On Tue, 2011-08-30 at 01:35 +0200, Frederic Weisbecker wrote:
> > That means it has to be in an extended grace period when we stop the
> > tick.
>
> You mean extended quiescent state?

Yeah that :-)

> As a summary here is what we do:
>
> - if we are in the kernel, we can't run into extended quiescent state because
> we may make use of rcu anytime there. But if we run nohz we don't have the tick
> to notice quiescent states to the RCU machinery and help completing grace periods
> so as soon as we receive an rcu IPI from another CPU (due to the grace period
> beeing extended because our nohz CPU doesn't report quiescent states), we restart
> the tick. We are optimistic enough to consider that we may avoid a lot of ticks
> even if there are some risks to be disturbed in some random rates.
> So even with the IPI we consider it as an upside.
>
> - if we are in userspace we can run in extended quiescent state.

But you can only disable the tick/enter extended quiescent state while
in kernel-space. Thus the second clause is precluded from ever being
true.