linux-kernel - Re: [PATCH v2] srcu: Isolate srcu sections using CONFIG

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Message-ID: <20150104161416.GA5280@linux.vnet.ibm.com>
Date:	Sun, 4 Jan 2015 08:14:16 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	Pranith Kumar <bobby.prani@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"supporter:S390" <linux390@...ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Scott Wood <scottwood@...escale.com>,
	Anton Blanchard <anton@...ba.org>,
	Daniel Walter <dwalter@...gle.com>,
	Tiejun Chen <tiejun.chen@...driver.com>,
	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Jens Freimann <jfrei@...ux.vnet.ibm.com>,
	"open list:LINUX FOR POWERPC..." <linuxppc-dev@...ts.ozlabs.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:S390" <linux-s390@...r.kernel.org>
Subject: Re: [PATCH v2] srcu: Isolate srcu sections using CONFIG_SRCU

On Sun, Jan 04, 2015 at 08:35:52PM +1100, Michael Ellerman wrote:
> On Tue, 2014-12-30 at 13:54 -0500, Pranith Kumar wrote:
> > On Tue, Dec 30, 2014 at 1:50 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Tue, Dec 30, 2014 at 12:46:22AM -0500, Pranith Kumar wrote:
> > >> Isolate the SRCU functions and data structures within CONFIG_SRCU so that there
> > >> is a compile time failure if srcu is used when not enabled. This was decided to
> > >> be better than waiting until link time for a failure to occur.
> > >
> > > Why?
> > 
> > This is part of the kernel tinification efforts. The first patch was
> > posted here: https://lkml.org/lkml/2014/12/4/848. This patch enables a
> > compile time failure instead of a link time failure.
> 
> The punch line was:
> 
>   "so the savings are about ~2000 bytes."
> 
> Which is utterly not worth the effort IMO. There have got to be more attractive
> targets for tinification than this.

There probably are.  But if the tinification effort is to come anywhere
near reaching its goals, it is going to need 2000-byte savings, especially
on the small systems that are this effort's main target.

That said, Peter's suggestion of falling back to the link-time diagnostic
does simplify things a bit, and might be a good approach.

							Thanx, Paul

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