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SubjectRe: [ANNOUNCE] 3.7-nohz1
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On Thu, 2012-12-20 at 19:32 +0100, Frederic Weisbecker wrote:
> Hi,
>

Nice work Frederic!

> So this is a new version of the nohz cpusets based on 3.7, except it's not using
> cpusets anymore and I actually based it on the middle of the 3.8 merge window
> in order to get latest upstream full dynticks preparatory work: cputime cleanups,
> RCU user mode, context tracking subsystem, nohz code consolidation, ...
>
> So the big changes since the last nohz cpuset release are:
>
> * printk now uses irq work so it doesn't rely on the tick anymore (provided
> your arch implements irq work with IPIs or alike). This chunk has been proposed
> for the 3.8 merge window: https://lkml.org/lkml/2012/12/17/177
> May be Linus will pull, may be not. We'll see. In any case I've included it in this tree
> but I'm not reposting this part of the patchset to avoid spamming you.
>
> * cputime doesn't rely on IPIs anymore. Now the reader does a special computation to
> remotely get the tickless cputime.
>
> * No more cpusets interface. Paul McKenney suggested me to start with a boot time
> kernel parameter to define the full dynticks cpumask. And he was totally right, it
> makes the code much more simple. That's a good way to start and to make the mainlining
> easier. We can still add a runtime configuration later if necessary.
>
> * Now there is always a CPU handling the timekeeping. This can be further optimized
> and more power-friendly, I really did something simple-stupid. I guess we'll try to get
> that into a better shape with Hakan. But at least the timekeeping now works.
>
> * It uses the new RCU callbacks offlining feature. This way a full dynticks CPU doesn't
> need to keep the tick to handle local callbacks. This is still very experimental though.
>
> * No more specific IPI vector for full dynticks. We just use the scheduler ipi.
>
> The branch is:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> 3.7-nohz1
>
> There is still quite some work to do.
>
> == How to use? ==
>
> Select:
> CONFIG_NO_HZ
> CONFIG_RCU_USER_QS
> CONFIG_VIRT_CPU_ACCOUNTING_GEN
> CONFIG_RCU_NOCB_CPU
> CONFIG_NO_HZ_FULL
>
> You always need at least one timekeeping CPU.
>
> Let's imagine you have 4 CPUs. We keep the CPU 0 to offline RCU callbacks there and to
> handle the timekeeping. We set the rest as full dynticks. So you need the following kernel
> parameters:
>
> rcu_nocbs=1-3 full_nohz=1-3
>
> (Note rcu_nocbs value must always be the same as full_nohz).

Why? You can't have: rcu_nocbs=1-4 full_nohz=1-3
or: rcu_nocbs=1-3 full_nohz=1-4 ?

That needs to be fixed. Either with a warning, and/or to force the two
to be the same. That is, if they specify:

rcu_nocbs=1-3 full_nohz=1-4

Then set rcu_nocbs=1-4 with a warning about it. Or simply set
full_nohz=1-3.

-- Steve

>
> Now if you want proper isolation you need to:
>
> * Migrate your processes adequately
> * Migrate your irqs to CPU 0
> * Migrate the RCU nocb threads to CPU 0. Example with the above configuration:
>
> for p in $(ps -o pid= -C rcuo1,rcuo2,rcuo3)
> do
> taskset -cp 0 $p
> done
>
> Then run what you want on the full dynticks CPUs. For best results, run 1 task
> per CPU, mostly in userspace and mostly CPU bound (otherwise more IO = more kernel
> mode execution = more chances to get IPIs, tick restarted, workqueues, kthreads, etc...)
>
> This page contains a good reminder for those interested in CPU isolation: https://github.com/gby/linux/wiki
>
> But keep in mind that my tree is not yet ready for serious production.
>




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