Messages in this thread | | | Date | Fri, 3 Apr 2009 00:36:05 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] v3 RCU: the bloatwatch edition |
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* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> This patch is a version of RCU designed for (!SMP && EMBEDDED) > provided as a proof of concept of a small-footprint RCU > implementation. In particular, the implementation of > synchronize_rcu() is extremely lightweight and high performance. > It passes rcutorture testing in each of the four relevant > configurations (combinations of NO_HZ and PREEMPT) on x86. This > saves about 900 bytes compared to Classic RCU, and a couple > kilobytes compared to Hierarchical RCU: > > CONFIG_CLASSIC_RCU: > > text data bss dec hex filename > 363 12 24 399 18f kernel/rcupdate.o > 1237 64 124 1425 591 kernel/rcuclassic.o > 1824 Total > > CONFIG_TREE_RCU: > > text data bss dec hex filename > 363 12 24 399 18f kernel/rcupdate.o > 2344 240 184 2768 ad0 kernel/rcutree.o > 3167 Total > > CONFIG_TINY_RCU: > > text data bss dec hex filename > 294 12 24 330 14a kernel/rcupdate.o > 563 36 0 599 257 kernel/rcutiny.o > 929 Total > > > Changes from v2 (http://lkml.org/lkml/2009/2/3/333) include: > > o Fix whitespace issues. > > o Change short-circuit "||" operator to instead be "+" in order to > fix performance bug noted by "kraai" on LWN. > > (http://lwn.net/Articles/324348/) > > Changes from v1 (http://lkml.org/lkml/2009/1/13/440) include: > > o This version depends on EMBEDDED as well as !SMP, as suggested > by Ingo. > > o Updated rcu_needs_cpu() to unconditionally return zero, > permitting the CPU to enter dynticks-idle mode at any time. > This works because callbacks can be invoked upon entry to > dynticks-idle mode. > > o I am now OK with this being included, based on a poll at the > Kernel Miniconf at linux.conf.au, where about ten people said > that they cared about saving 900 bytes on single-CPU systems. > > o Applies to both mainline and tip/core/rcu.
Andrew, what do you think?
A worry is yet another RCU variant - we already have 3.
A trick we could use would be to put it into Documentation/rcu/, linked in via some clever Makefile magic and only usable if a ultra-embedded developer does a build with something like CONFIG_RCU_TINY=y. That way there's no real maintenance and testing overhead.
It _does_ have documentation value beyond the ~900 bytes: it's the simplest and smallest possible still-working UP RCU implementation so it would be easy to teach RCU concepts via that, gradually.
Ingo
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