Re: [PATCH] v3 RCU: the bloatwatch edition

[Paul Mundt <lethal@linux-sh.org>]

On Fri, Apr 03, 2009 at 12:36:05AM +0200, Ingo Molnar wrote:
> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > This patch is a version of RCU designed for (!SMP && EMBEDDED) 
> > provided as a proof of concept of a small-footprint RCU 
> > implementation. In particular, the implementation of 
> > synchronize_rcu() is extremely lightweight and high performance.  
> > It passes rcutorture testing in each of the four relevant 
> > configurations (combinations of NO_HZ and PREEMPT) on x86.  This 
> > saves about 900 bytes compared to Classic RCU, and a couple 
> > kilobytes compared to Hierarchical RCU:
> 
> Andrew, what do you think?
> 
> A worry is yet another RCU variant - we already have 3.
> 
> A trick we could use would be to put it into Documentation/rcu/, 
> linked in via some clever Makefile magic and only usable if a 
> ultra-embedded developer does a build with something like 
> CONFIG_RCU_TINY=y. That way there's no real maintenance and testing 
> overhead.
> 
> It _does_ have documentation value beyond the ~900 bytes: it's the 
> simplest and smallest possible still-working UP RCU implementation 
> so it would be easy to teach RCU concepts via that, gradually.
> 
A similar argument could have been used for tiny-shmem when it was first
integrated. As this is hiding behind CONFIG_EMBEDDED, most users are not
going to run in to it, so the confusion of 1 more RCU variant is not
likely to be a problem for those that aren't actively seeking it out.

So, personally I think it is a good idea, and I have no reservations
about default enabling it for a number of more constrained SH platforms.