linux-kernel - Re: [GIT PULL] isolation: 1Hz residual tick offloading v3

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Date:   Wed, 17 Jan 2018 08:51:13 -0600 (CST)
From:   Christopher Lameter <cl@...ux.com>
To:     Mike Galbraith <efault@....de>
cc:     Frederic Weisbecker <frederic@...nel.org>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [GIT PULL] isolation: 1Hz residual tick offloading v3

On Tue, 16 Jan 2018, Mike Galbraith wrote:

> > I tried to remove isolcpus or at least change the way it works so that its
> > effects are reversible (ie: affine the init task instead of isolating domains)
> > but that got nacked due to the behaviour's expectations for userspace.
>
> So we paint ourselves into a static corner forever more, despite every
> bit of this being all about "properties of sets of cpus", ie precisely
> what cpusets was born to do.  That's sad, dynamic wasn't that far away.

cpusets was born in order to isolate applications to sets of processors.
The properties of sets of cpus was not on the horizon when SGI started
this.

We have sets of cpus associated with affinity masks in the form of bitmaps
etc etc which is much more lightweight than having slug around the cgroup
overhead everywhere.

A simple bitmask is much better if you have to control detailed system
behavior for each core and are planning each processes role because you
need to make full use of the harware resources available.