linux-kernel - Re: [PATCH] v3 RCU: the bloatwatch edition

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Message-ID: <20090402224455.GA13777@linux-sh.org>
Date:	Fri, 3 Apr 2009 07:44:56 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, niv@...ibm.com, dvhltc@...ibm.com,
	dhowells@...hat.com, kernel@...tstofly.org, matthew@....cx
Subject: Re: [PATCH] v3 RCU: the bloatwatch edition

On Fri, Apr 03, 2009 at 12:36:05AM +0200, Ingo Molnar wrote:
> * Paul E. McKenney <paulmck@...ux.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.
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