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

Frederic Weisbecker
2011-08-30T14:32:15Z

On Tue, Aug 30, 2011 at 01:21:55PM +0200, Peter Zijlstra wrote:
> 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.

No, we have a specific stacking in the irq:

rcu_irq_enter()

disable tick...
if (user)
rcu_enter_nohz();

rcu_irq_exit() <-- extended quiescent state entry effective only there

And by the time we call rcu_irq_exit() and we resume to userspace, we are
not supposed to have rcu read side critical section (minus the case of
a signal with do_notify_resume() which I have yet to handle).