Re: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ_FULL is enabled

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 16, 2013 at 08:07:06AM -0700, Paul E. McKenney wrote:
> On Thu, May 16, 2013 at 10:10:27AM +0200, Peter Zijlstra wrote:
> > On Wed, May 15, 2013 at 01:04:01PM -0400, Steven Rostedt wrote:
> > > On Wed, 2013-05-15 at 18:59 +0200, Peter Zijlstra wrote:
> > > 
> > > > At which point we could run the watchdog without perf_event_task_tick().
> > > 
> > > At which point we can drop the disable LOCKUP_DETECTOR when NO_HZ_FULL
> > > is enabled ;-)
> > > 
> > 
> > Can we? The thing I'm worried about is RCU (of course!). ISTR we rely on RCU
> > working in NMI context. AFAIR for RCU to work, we need to come out of out magic
> > NO_HZ state since that would've put RCU into EQS.
> > 
> > Frederic, PaulMck?
> 
> Not sure I understand the question, but hopefully the verbiage below helps.
> 
> Only RCU read-side critical sections need to work in NMI context,
> and RCU hooks into nmi_enter() and nmi_exit() to handle this, and this
> will work in NO_HZ_FULL in the same way that it works for NO_HZ_IDLE.
> 
> But if there are no NMIs, RCU doesn't care.  In other words, RCU needs
> to know about NMIs so that it can deal with any RCU read-side critical
> sections in the NMI handlers, but RCU doesn't rely on NMIs happening at
> any particular time or frequency.

I suppose the fundamental question was: will receiving NMIs negate NO_HZ_FULL's
functionality? That is, will the getting of NMIs make us drop out of NO_HZ_FULL
and re-enable all sorts of things?

Because clearly RCU needs to exit from EQS, which might (or might not) mean
leaving NO_HZ_FULL.

I'm not entirely up-to-date on those details.