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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, May 16, 2013 at 07:56:02PM +0200, Peter Zijlstra wrote:
> 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.

My belief is that NMIs won't cause NO_HZ_FULL to kick that CPU out of
adaptive-ticks mode, but I must defer to Frederic on that.

Of course, the NMI -will- cause OS jitter on whichever CPU handles it,
which some people would want to avoid.

							Thanx, Paul