Re: [PATCH 4/4] nohz: New option to force all CPUs in full dynticks range

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Apr 11, 2013 at 10:11:53AM -0700, Paul E. McKenney wrote:
> On Thu, Apr 11, 2013 at 07:04:23PM +0200, Frederic Weisbecker wrote:
> > On Thu, Apr 11, 2013 at 04:41:07PM +0000, Christoph Lameter wrote:
> > > On Thu, 11 Apr 2013, Frederic Weisbecker wrote:
> > > 
> > > > If there is no performance issue with that I'm all for it.
> > > 
> > > Or have a
> > > 
> > > CONFIG_LOWLATENCY
> > > 
> > > that boots up a kernel with the proper configuration?
> > 
> > It may be too general for a naming. But I don't mind just
> > selecting CONFIG_RCU_NOCBS_ALL unconditionally. It's easily
> > changed in the future if anybody complains.
> > 
> > Btw, if CONFIG_RCU_NOCBS_ALL is set, the rcu_nocbs= parameter
> > is ignored, right? If you want to keep that direction and not
> > override the Kconfig choice, may be warn the user about that
> > if the boot parameter is passed?
> 
> Fair point.  Let me think about how I should approach Thanx.

An alternative is to treat the Kconfig choice as a default setting
that gets overriden by rcu_nocbs=

This would require to refactor the three way Kconfig layout you've set,
but that aligns with what Ingo is suggesting me to do with the full dynticks
range: having CONFIG_NO_HZ_EXTENDED_ALL that is overriden by nohz_extended=
if any (beware though, "extended" will soon be renamed to "full", lets hope
it's our last take ;)

That unifies both behaviours and it looks more flexible to me.

Of course that means selecting CONFIG_RCU_NOCBS_ALL won't be enough for
me to ensure my nohz range is also nocb, I'm fine with the cpumask check
on boot though.

Anyway, that's just a suggestion, the most important is that the
user is informed of what's happening and how to deal with it.