linux-kernel - Re: [PATCH] v4 RCU: the bloatwatch edition

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Message-ID: <20090506190216.GE6771@linux.vnet.ibm.com>
Date:	Wed, 6 May 2009 12:02:16 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, dhowells@...hat.com,
	linux-kernel@...r.kernel.org, niv@...ibm.com, dvhltc@...ibm.com,
	lethal@...ux-sh.org, kernel@...tstofly.org, matthew@....cx
Subject: Re: [PATCH] v4 RCU: the bloatwatch edition

On Wed, May 06, 2009 at 11:24:59AM -0700, Andrew Morton wrote:
> On Wed, 6 May 2009 14:09:09 +0200
> Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > * David Howells <dhowells@...hat.com> 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 (updated to 2.6.29):
> > > > ...
> > > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > 
> > > Acked-by: David Howells <dhowells@...hat.com>
> > 
> > i'm wondering what Andrew thinks - he had objections, right?
> > 
> 
> More like "concerns".  It's unobvious to me that the modest .text
> savings justify the costs of an additional RCU implementation.  Where
> those costs include
> 
> - additional maintenance work and
> 
> - the reduced code reliability which comes from fragmenting the
>   tester base.  This will mostly affect users of the less popular RCU
>   implementations.
> 
> But hey, maybe I'm wrong.  And maybe I'm right, but we'll merge it anyway ;)

;-)

How about if acceptance of Tiny RCU happens at the same time as Classic
RCU is dropped?  That would be a large net decrease in code size and
complexity.

							Thanx, Paul
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