Re: [PATCH RFC tip/core/rcu 19/28] nohz: Allow rcu extended quiescent state handling seperately from tick stop

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Nov 03, 2011 at 09:06:56AM -0700, Paul E. McKenney wrote:
> On Thu, Nov 03, 2011 at 08:31:02AM -0700, Josh Triplett wrote:
> > On Thu, Nov 03, 2011 at 06:32:31AM -0700, Paul E. McKenney wrote:
> > > On Thu, Nov 03, 2011 at 12:54:33PM +0100, Frederic Weisbecker wrote:
> > > > On Wed, Nov 02, 2011 at 09:00:03PM -0700, Josh Triplett wrote:
> > > > > On Wed, Nov 02, 2011 at 01:30:40PM -0700, Paul E. McKenney wrote:
> > > > > > From: Frederic Weisbecker <fweisbec@gmail.com>
> > > > > > 
> > > > > > It is assumed that rcu won't be used once we switch to tickless
> > > > > > mode and until we restart the tick. However this is not always
> > > > > > true, as in x86-64 where we dereference the idle notifiers after
> > > > > > the tick is stopped.
> > > > > > 
> > > > > > To prepare for fixing this, add two new APIs:
> > > > > > tick_nohz_idle_enter_norcu() and tick_nohz_idle_exit_norcu().
> > > > > > 
> > > > > > If no use of RCU is made in the idle loop between
> > > > > > tick_nohz_enter_idle() and tick_nohz_exit_idle() calls, the arch
> > > > > > must instead call the new *_norcu() version such that the arch doesn't
> > > > > > need to call rcu_idle_enter() and rcu_idle_exit().
> > > > > 
> > > > > The _norcu names confused me a bit.  At first, I thought they meant
> > > > > "idle but not RCU idle, so you can use RCU", but from re-reading the
> > > > > commit message, apparently they mean "idle and RCU idle, so don't use
> > > > > RCU".  What about something like _forbid_rcu instead?  Or,
> > > > > alternatively, why not just go ahead and separate the two types of idle
> > > > > entirely rather than introducing the _norcu variants first?
> > > > 
> > > > Or tick_nohz_idle_enter_rcu_stop() and tick_nohz_idle_exit_rcu_restart()?
> > > > 
> > > > Sounds clear but too long. May be we can shorten the tick_nohz thing in the
> > > > beginning.
> > > 
> > > How about tick_nohz_rcu_idle_enter() vs. tick_nohz_idle_enter() on
> > > entry to the idle loop and tick_nohz_rcu_idle_exit() vs
> > > tick_nohz_idle_exit() on exit?
> > > 
> > > That said, I don't feel all that strongly on this naming topic.
> > 
> > Mostly I think that since this series tries to separate the concepts of
> > "idle nohz" and "rcu extended quiescent state", we should end up with
> > two entirely separate functions delimiting those two, without any
> > functions that poke both with correspondingly complex compound names.
> 
> Having four API members rather than the current six does seem quite
> attractive to me.  Frederic, any reason why this approach won't work?

The approach I took might sound silly but it's mostly an optimization:

I did the *_norcu() variant mostly to be able to keep rcu_idle_enter()
call under the same local_irq_disable() section.

This way we can't have an interrupt in between that can needlessly perform
RCU work (and trigger the softirq in the worst case), delaying the point
where we actually put the CPU to sleep.