Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU

Paul E. McKenney
2010-01-27T05:21:39Z

On Tue, Jan 26, 2010 at 06:55:16PM -0500, Mathieu Desnoyers wrote:
> * Andi Kleen (andi@firstfloor.org) wrote:
> > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
> >
> > Kind of offtopic to the original patch, but I couldn't
> > resist...
> >
> > > +config RCU_FAST_NO_HZ
> > > + bool "Accelerate last non-dyntick-idle CPU's grace periods"
> > > + depends on TREE_RCU && NO_HZ && SMP
> >
> > Having such a thing as a config option doesn't really make
> > any sense to me. Who would want to recompile their kernel
> > to enable/disable this? If anything it should be runtime, or better
> > just unconditionally on.
> >
> > In general RCU could probably reduce its number of "weird"
> > Kconfig options.
> >
> > While I think I have a better understanding of RCU than a lot
> > of normal users I often have no clue what to set there when
> > building a kernel.
>
> Maybe we could keep them under a CONFIG_DEBUG_RCU umbrella. Compiling
> out parts of the rcu options can be useful for debugging purposes, but
> I agree with you that end users should not be bothered with that much
> options when some of them are "obviously" wanted.
>
> OTOH, I understand that Paul seems to want to introduce new RCU
> features gradually, without hitting all kernel users with bugs in newer
> features. That's a sane approach to keep things generally stable, but
> maybe it is time to set some of the stabilized RCU options to default Y
> and move their config to the debug menu.

That is indeed a big part of the motivation. ;-)

Thanx, Paul

> Let's see what Paul has to say about this...
>
> Thanks,
>
> Mathieu
>
> >
> > -Andi
> >
> > --
> > ak@linux.intel.com -- Speaking for myself only.
>
> --
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68