linux-kernel - Re: [ANNOUNCE] 3.7-nohz1

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Date:	Thu, 20 Dec 2012 21:35:32 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Li Zhong <zhong@...ux.vnet.ibm.com>
Subject: Re: [ANNOUNCE] 3.7-nohz1

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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