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Date: Sun, 4 Jan 2015 08:14:16 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH v2] srcu: Isolate srcu sections using CONFIG_SRCU
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To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Peter Zijlstra <peterz@infradead.org>, Heiko Carstens <heiko.carstens@de.ibm.com>, Tiejun Chen <tiejun.chen@windriver.com>, Paul Mackerras <paulus@samba.org>, Daniel Walter <dwalter@google.com>, Ingo Molnar <mingo@kernel.org>, "open list:S390" <linux-s390@vger.kernel.org>, Vincent Guittot <vincent.guittot@linaro.org>, Christian Borntraeger <borntraeger@de.ibm.com>, Jens Freimann <jfrei@linux.vnet.ibm.com>, Pranith Kumar <bobby.prani@gmail.com>, Josh Triplett <josh@joshtriplett.org>, Steven Rostedt <rostedt@goodmis.org>, Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, Anton Blanchard <anton@samba.org>, Scott Wood <scottwood@freescale.com>, Lai Jiangshan <laijs@cn.fujitsu.com>, Li Zhong <zhong@linux.vnet.ibm.com>, Nishanth Aravamudan <nacc@linux.vnet.ibm.com>, open list <linux-kernel@vger.kernel.org>, "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>, Martin Schwidefsky <schwidefsky@de.ibm.com>, "supporter:S390" <linux390@de.ibm.com>, "open list:LINUX FOR POWERPC..." <linuxppc-dev@lists.ozlabs.org>
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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@infradead.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