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 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.