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Re: [microblaze-uclinux] Re: [PATCH 6/6] Add support for __read_mostly t

To: Christoph Lameter <cl@linux.com>
Subject: Re: [microblaze-uclinux] Re: [PATCH 6/6] Add support for __read_mostly to linux/cache.h
From: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Date: Fri, 1 May 2009 23:47:13 +0200
Cc: Sam Ravnborg <sam@ravnborg.org>, Tim Abbott <tabbott@MIT.EDU>, Anders Kaseorg <andersk@MIT.EDU>, Waseem Daher <wdaher@MIT.EDU>, Denys Vlasenko <vda.linux@googlemail.com>, Jeff Arnold <jbarnold@MIT.EDU>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Bryan Wu <cooloney@kernel.org>, Chris Zankel <chris@zankel.net>, Cyrill Gorcunov <gorcunov@openvz.org>, David Howells <dhowells@redhat.com>, "David S. Miller" <davem@davemloft.net>, dev-etrax@axis.com, Geert Uytterhoeven <geert@linux-m68k.org>, Greg Ungerer <gerg@uclinux.org>, Haavard Skinnemoen <hskinnemoen@atmel.com>, Heiko Carstens <heiko.carstens@de.ibm.com>, Helge Deller <deller@gmx.de>, Hirokazu Takata <takata@linux-m32r.org>, "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>, Jeff Dike <jdike@addtoit.com>, Jesper Nilsson <jesper.nilsson@axis.com>, Kyle McMartin <kyle@mcmartin.ca>, Linus Torvalds <torvalds@linux-foundation.org>, linux-alpha@vger.kernel.org, linux-am33-list@redhat.com, linux-arm-kernel@lists.arm.linux.org.uk, linux-ia64@vger.kernel.org, linux-m32r@ml.linux-m32r.org, linux-m68k@vger.kernel.org, linux-mips@linux-mips.org, linux-parisc@vger.kernel.org, linuxppc-dev@ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, Martin Schwidefsky <schwidefsky@de.ibm.com>, Michal Simek <monstr@monstr.eu>, microblaze-uclinux@itee.uq.edu.au, Mikael Starvik <starvik@axis.com>, Paul Mackerras <paulus@samba.org>, Paul Mundt <lethal@linux-sh.org>, Ralf Baechle <ralf@linux-mips.org>, Richard Henderson <rth@twiddle.net>, Roman Zippel <zippel@linux-m68k.org>, Russell King <rmk+kernel@arm.linux.org.uk>, sparclinux@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>, Tony Luck <tony.luck@intel.com>, uclinux-dist-devel@blackfin.uclinux.org, user-mode-linux-devel@lists.sourceforge.net, Yoshinori Sato <ysato@users.sourceforge.jp>
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On Fri, May 01, 2009 at 09:52:18AM -0400, Christoph Lameter wrote:
> On Fri, 1 May 2009, Sam Ravnborg wrote:
> 
> > Are there any specific reason why we do not support read_mostly on all
> > architectures?
> 
> Not that I know of.
> 
> > read_mostly is about grouping rarely written data together
> > so what is needed is to introduce this section in the remaining
> > archtectures.
> >
> > Christoph - git log says you did the inital implmentation.
> > Do you agree?
> 
> Yes.
> 
> There is some concern that __read_mostly is needlessly applied to
> numerous variables that are not used in hot code paths. This may make
> __read_mostly ineffective and actually increase the cache footprint of a
> function since global variables are no longer in the same cacheline. If
> such a function is called and the caches are cold then two cacheline
> fetches have to be done instead of one.

FWIW I think that's a valid concern. Also, I think one can question the
value of __read_mostly for write-through caches, given the mentioned
concern it probably makes things worse for those. IMO there should be
a way to turn it off for arch's that know it's no good for them.

Cheers

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