Date: Wed, 8 Apr 2009 18:38:38 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Paul Mundt <lethal@linux-sh.org>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
       niv@us.ibm.com, dvhltc@us.ibm.com, dhowells@redhat.com,
       kernel@wantstofly.org, matthew@wil.cx, Matt Mackall <mpm@selenic.com>
Subject: Re: [PATCH] v3 RCU: the bloatwatch edition
Message-ID: <20090408163838.GB14449@elte.hu>
References: <20090203183426.GA14409@linux.vnet.ibm.com> <20090329203118.GA14005@linux.vnet.ibm.com> <20090402223605.GB19007@elte.hu> <20090402224455.GA13777@linux-sh.org>
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* Paul Mundt <lethal@linux-sh.org> wrote:

> 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.

but at least tiny-shmem is now nicely hidden in mm/shmem.c, in an 
unintrusive !CONFIG_SHMEM branch. There's no CONFIG_TINY_SHMEM 
option anymore - it's all done in the !CONFIG_SHMEM case.

Is tiny-RCU in the same category?

	Ingo
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