linux-kernel - Re: [RFC PATCH 1/8] Convert the RCU tasklet into a softirq

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Date:	Mon, 16 Jul 2007 22:48:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@...radead.org>,
	john stultz <johnstul@...ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>, kuznet@....inr.ac.ru,
	Jonathan Corbet <corbet@....net>,
	Arjan van de Ven <arjan@...radead.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC PATCH 1/8] Convert the RCU tasklet into a softirq


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Mon, 16 Jul 2007, Paul E. McKenney wrote:
> >
> > On Wed, Jun 27, 2007 at 03:36:34PM -0400, Steven Rostedt wrote:
> > > I believe this was originally done by Dipankar Sarma. I pulled these
> > > changes from the -rt kernel.
> > > 
> > > For better preformance, RCU should use a softirq instead of a
> > > tasklet.
> > 
> > Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Tested-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > Tested with both kernbench and rcutorture on i386, x86_64, and ppc64.
> 
> Btw, are there any numbers on this? Is this whole thing actually 
> noticeable?

i doubt it's noticeable, because the RCU tasklet was not a "true" global 
tasklet, it was an array of tasklets put into a PER_CPU array. (which 
kind of defeats the purpose of tasklets)

So there should be no/little scalability disadvantage from this type of 
use of tasklets, and the tasklet use was at most a wart and a small, 
constant micro-cost that is likely masked by other costs. (and rcu never 
used any of the fancier tasklet ops like tasklet_disable() either)

	Ingo
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