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SubjectRe: [PATCH -tip] v10 scalable classic RCU implementation
On Tue, 16 Dec 2008 16:37:44 -0800 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> Hello!
>
> This patch fixes a long-standing performance bug in classic RCU that
> results in massive internal-to-RCU lock contention on systems with
> more than a few hundred CPUs. Although this patch creates a separate
> flavor of RCU for ease of review and patch maintenance, it is intended
> to replace classic RCU.
>
> This patch still handles stress better than does mainline, so I am still
> calling it ready for inclusion. This patch is against the -tip tree.
> Nevertheless, experience on an actual 1000+ CPU machine would still be
> most welcome.
>
> Most of the changes noted below were found while creating an rcutiny
> (which should permit ejecting the current rcuclassic) and while doing
> detailed line-by-line documentation.
>
> Updates from v9 (http://lkml.org/lkml/2008/12/2/334):
>
> o Fixes from remainder of line-by-line code walkthrough,
> including comment spelling, initialization, undesirable
> narrowing due to type conversion, removing redundant memory
> barriers, removing redundant local-variable initialization,
> and removing redundant local variables.
>
> I do not believe that any of these fixes address the CPU-hotplug
> issues that Andi Kleen was seeing, but please do give it a whirl
> in case the machine is smarter than I am.
>
> A writeup from the walkthrough may be found at the following
> URL, in case you are suffering from terminal insomnia or
> masochism:
>
> http://www.kernel.org/pub/linux/kernel/people/paulmck/tmp/rcutree-walkthrough.2008.12.16a.pdf
>
> o Made rcutree tracing use seq_file, as suggested some time
> ago by Lai Jiangshan.
>
> o Added a .csv variant of the rcudata debugfs trace file, to allow
> people having thousands of CPUs to drop the data into
> a spreadsheet. Tested with oocalc and gnumeric. Updated
> documentation to suit.
>
> Updates from v8 (http://lkml.org/lkml/2008/11/15/139):
> Updates from v7 (http://lkml.org/lkml/2008/10/10/291):
> Updates from v6 (http://lkml.org/lkml/2008/9/23/448):
> Updates from v5 (http://lkml.org/lkml/2008/9/15/92, bad subject line):
> Updates from v4 for those missing v5 given its bad subject line:

It strikes me that this code is at the level of maturity where it
should have been in linux-next months ago.

>
> To build, start with 2.6.28-rc3, and apply:
>
> http://www.rdrop.com/users/paulmck/patches/linux-2.6-tip.2008.11.29a.patch
> http://www.rdrop.com/users/paulmck/patches/2.6.28-rc3-tip-treeRCU-7.patch
>
> The former gets you to commit #29cbda77a67cf263d636feea65d3bbc9c7de2e24
> in /tip/core/rcu in the -tip tree, and the latter is the following patch.
>
> Thoughts?

Merge it?

> +choice
> + prompt "RCU Implementation"
> + default CLASSIC_RCU

But toggle that.

> +config CLASSIC_RCU
> + bool "Classic RCU"
> + help
> + This option selects the classic RCU implementation that is
> + designed for best read-side performance on non-realtime
> + systems.
> +
> + Select this option if you are unsure.
> +
> +config TREE_RCU
> + bool "Tree-based hierarchical RCU"
> + help
> + This option selects the RCU implementation that is
> + designed for very large SMP system with hundreds or
> + thousands of CPUs.
> +

If it blows up, we can trivially disable it in config, yes?

> Documentation/RCU/00-INDEX | 2
> Documentation/RCU/trace.txt | 413 ++++++++
> arch/powerpc/platforms/pseries/rtasd.c | 4
> include/linux/hardirq.h | 14
> include/linux/rcupdate.h | 10
> include/linux/rcutree.h | 329 +++++++
> init/Kconfig | 18
> kernel/Kconfig.preempt | 62 +
> kernel/Makefile | 6
> kernel/rcupreempt.c | 10
> kernel/rcupreempt_trace.c | 10
> kernel/rcutree.c | 1535 +++++++++++++++++++++++++++++++++
> kernel/rcutree_trace.c | 271 +++++
> kernel/softirq.c | 5
> lib/Kconfig.debug | 13
> 15 files changed, 2671 insertions(+), 31 deletions(-)

What's the plan for removal of the classic RCU code? (asap, please.
No more slabs!)



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