From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH RFC] Simplify the Linux kernel by reducing its state space Date: Sun, 1 Apr 2012 11:12:11 -0700 Message-ID: <20120401181211.GZ2450@linux.vnet.ibm.com> References: <20120331163321.GA15809@linux.vnet.ibm.com> <20120331223200.GA32482@n2100.arm.linux.org.uk> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: linux-arm-kernel-bounces@lists.infradead.org Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org To: Thomas Gleixner Cc: linux-m32r-ja@ml.linux-m32r.org, linux-mips@linux-mips.org, linux-ia64@vger.kernel.org, linux-m32r@ml.linux-m32r.org, Russell King - ARM Linux , linux-parisc@vger.kernel.org, linux-s390@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-sh@vger.kernel.org, linux-kernel@vger.kernel.org, cmetcalf@tilera.com, jejb@parisc-linux.org, dhowells@redhat.com, linux390@de.ibm.com, linux-am33-list@redhat.com, linux-alpha@vger.kernel.org, sparclinux@vger.kernel.org, uclinux-dist-devel@blackfin.uclinux.org, x86@kernel.org, linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org On Sun, Apr 01, 2012 at 07:34:55PM +0200, Thomas Gleixner wrote: > On Sat, 31 Mar 2012, Russell King - ARM Linux wrote: > > > On Sun, Apr 01, 2012 at 12:33:21AM +0800, Paul E. McKenney wrote: > > > Although there have been numerous complaints about the complexity of > > > parallel programming (especially over the past 5-10 years), the plain > > > truth is that the incremental complexity of parallel programming over > > > that of sequential programming is not as large as is commonly believed. > > > Despite that you might have heard, the mind-numbing complexity of modern > > > computer systems is not due so much to there being multiple CPUs, but > > > rather to there being any CPUs at all. In short, for the ultimate in > > > computer-system simplicity, the optimal choice is NR_CPUS=0. > > > > > > This commit therefore limits kernel builds to zero CPUs. This change > > > has the beneficial side effect of rendering all kernel bugs harmless. > > > Furthermore, this commit enables additional beneficial changes, for > > > example, the removal of those parts of the kernel that are not needed > > > when there are zero CPUs. > > > > > > Signed-off-by: Paul E. McKenney > > > Reviewed-by: Thomas Gleixner > > > > Great work, but I don't think you've gone far enough with this. > > > > What would really help is if you could consolidate all these NR_CPUS > > definitions into one place so we don't have essentially the same thing > > scattered across all these architectures. We're already doing this on > > ARM across our platforms, and its about time such an approach was taken > > across the entire kernel tree. > > > > It looks like the MIPS solution would be the best one to pick. > > Could you rework your patch to do this please? > > > > While you're at it, you might like to consider that having zero CPUs > > makes all this architecture support redundant, so maybe you've missed > > a trick there - according to my count, we could get rid of almost 3 > > million lines of code from arch. We could replace all that with a > > single standard implementation. > > For a first step we can deprecated arch/ and make it depend on > CONFIG_STAGING. That way we can have it around a bit for sentimental > reasons w/o having a lot of churn. > > Suggested-by: Russell King > Signed-off-by: Thomas Gleixner ;-) ;-) ;-) Reviewed-by: Paul E. McKenney Thanx, Paul > Index: tip/Makefile > =================================================================== > --- tip.orig/Makefile > +++ tip/Makefile > @@ -564,7 +564,9 @@ else > KBUILD_CFLAGS += -O2 > endif > > +ifdef CONFIG_ARCH_SUPPORT > include $(srctree)/arch/$(SRCARCH)/Makefile > +endif > > ifneq ($(CONFIG_FRAME_WARN),0) > KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN}) > Index: tip/drivers/staging/Kconfig > =================================================================== > --- tip.orig/drivers/staging/Kconfig > +++ tip/drivers/staging/Kconfig > @@ -1,6 +1,10 @@ > +config ARCH_SUPPORT > + bool > + > menuconfig STAGING > bool "Staging drivers" > default n > + select ARCH_SUPPORT > ---help--- > This option allows you to select a number of drivers that are > not of the "normal" Linux kernel quality level. These drivers > Index: tip/Documentation/feature-removal-schedule.txt > =================================================================== > --- tip.orig/Documentation/feature-removal-schedule.txt > +++ tip/Documentation/feature-removal-schedule.txt > @@ -537,3 +537,13 @@ When: 3.6 > Why: setitimer is not returning -EFAULT if user pointer is NULL. This > violates the spec. > Who: Sasikantha Babu > + > +----------------------------- > + > +What: Remove arch > +When: April 1st 2013 > +Why: NR_CPUS=0 made arch/ obsolete. Keep it around a bit for > + sentimental reasons. > +Who: paulmck,tglx.rmk > + > + >