linux-kernel - Re: [PATCH 00/32] [RFC] nohz/cpuset: Start discussions on nohz CPUs

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Date:	Fri, 02 Nov 2012 10:37:25 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <clark.williams@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 00/32] [RFC] nohz/cpuset: Start discussions on nohz CPUs

On Fri, 2012-11-02 at 14:23 +0000, Christoph Lameter wrote:
> On Mon, 29 Oct 2012, Steven Rostedt wrote:
> 
> > A while ago Frederic posted a series of patches to get an idea on
> > how to implement nohz cpusets. Where you can add a task to a cpuset
> > and mark the set to be 'nohz'. When the task runs on a CPU and is
> > the only task scheduled (nr_running == 1), the tick will stop.
> > The idea is to give the task the least amount of kernel interference
> > as possible. If the task doesn't do any system calls (and possibly
> > even if it does), no timer interrupt will bother it. By using
> > isocpus and nohz cpuset, a task would be able to achieve true cpu
> > isolation.
> 
> I thought isolcpus was on the way out? If there is no timer interrupt then
> there will also be no scheduler activity. Why do we need both?

I probably shouldn't have mentioned isolcpus. I was using that as
something that is general to get everything off of a cpu (irq affinity
for example).

> 
> Also could we have this support without cpusets? There are multiple means
> to do system segmentation (f.e. cgroups) and something like hz control is
> pretty basic. Control via some cpumask like irq affinities in f.e.
> 
> 	/sys/devices/system/cpu/nohz
> 
> or a per cpu flag in
> 
> /sys/devices/system/cpu/cpu0/hz
> 
> would be easier and not be tied to something like cpusets.

Frederic will have to answer this. I was just starting with his patches.
Note, we are holding off this work for now until Frederic's other work
is done (the irq_work and printk updates).

> 
> also it would be best to sync this conceptually with the processors
> enabled for rcu processing.

Processors can be disabled for rcu processing? Or are you talking about
Paul's new work of offloading rcu callbacks?

> 
> Maybe have a series of cpumasks in /sys/devices/system/cpu/ ?
> 
> > This has been long asked for by those in the RT community. If a task
> > requires uninterruptible CPU time, this would be able to give a task
> > that, even without the full PREEMPT-RT patch set.
> 
> Also those interested in low latency are very very interested in this
> feature in particular in support without any preempt support on in the
> kernel.
> 

Yep understood. We really need to get things rolling.

-- Steve


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