linux-kernel - Re: [PATCH RFC tip/core/rcu 17/41] rcu: Remove single-rcu_node optimization in rcu_start

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Date:	Thu, 2 Feb 2012 09:16:43 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org
Subject: Re: [PATCH RFC tip/core/rcu 17/41] rcu: Remove single-rcu_node
 optimization in rcu_start_gp()

On Wed, Feb 01, 2012 at 06:13:14PM -0800, Josh Triplett wrote:
> On Wed, Feb 01, 2012 at 11:41:35AM -0800, Paul E. McKenney wrote:
> > The grace-period initialization sequence in rcu_start_gp() has a special
> > case for systems where the rcu_node tree is a single rcu_node structure.
> > This made sense some years ago when systems were smaller and up to 64
> > CPUs could share a single rcu_node structure, but now that large systems
> > are common and a given leaf rcu_node structure can support only 16 CPUs
> > (due to lock contention on the rcu_node's ->lock field), this optimization
> > is almost never taken.  And even the small mobile platforms that might
> > make use of it might rather have the kernel text reduction.
> > 
> > Therefore, this commit removes the check for single-rcu_node trees.
> 
> This optimization would continue to work on laptops for a while longer.
> :)

How many more months?  ;-)

> That said, I do agree that reducing code size and complexity seems
> preferable.  If someone wants an optimization like this, they'd probably
> do better to compile RCU with a low compile-time limit on the number of
> CPUs, which would at least theoretically allow the compiler to get
> similar results through optimization.  (I don't know if that works in
> practice with the current code structure and the current intelligence of
> GCC.)
> 
> Reviewed-by: Josh Triplett <josh@...htriplett.org>

Thank you for all your reviews -- as always, very helpful!!!

							Thanx, Paul

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