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

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Date:	Mon, 5 Nov 2012 23:32:20 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, 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>,
	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

2012/11/2 Christoph Lameter <cl@...ux.com>:
> 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.

You really don't want that cpuset interface, do you? ;-)

Yeah I think I agree with you. This adds a dependency to
cpusets/cgroups, I wish we could avoid that if possible. Also cpuset
may be a bit counter intuitive for this usecase. What if a cpu is
included in both a nohz cpuset and a non-nohz cpuset? What is the
behaviour to adopt? An OR on the nohz flag such that as long as the
CPU is in at least one nohz cpuset, it's considered a nohz CPU? Or
only shutdown the tick for the tasks attached in the nohz cpusets? Do
we really want that per cgroup granularity and the overhead /
complexity that comes along?

No I think we should stay simple and have a simple per CPU property
for that, without involving cgroups aside.

So indeed a cpumask in /sys/devices/system/cpu/nohz looks like a
better interface.

>> 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.

Sure, we are trying to make that full dyncticks approach as much
generic as possible.
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