Re: [PATCH V2] cpuset: Add knob to make allowed masks hotplug invariant on legacy hierarchy

From: Serge E. Hallyn
Date: Wed Apr 15 2015 - 12:36:37 EST

On Wed, Apr 15, 2015 at 12:18:11PM -0400, Tejun Heo wrote:
> On Wed, Apr 15, 2015 at 11:15:35AM -0500, Serge E. Hallyn wrote:
> > The reason would be because it breaks "legacy" software. So that
> > would only matter if Preeti needs to run such software.
> Sure, I get that argument but this is changing how the contorller
> behaves in a major way.

It is. My main counter to that would be that it is how cpusets
should always have worked :)

> There are specifics which may make this
> particular case more justifiable but overall the combination of
> arguments is pretty weird.

And becomes harder to reason about and review/maintain. I agree

>From userspace, I suppose one approach (though note it is racy) to
solving this would be to have udev rules which

. On cpu unplug, record all cgroups which were using that cpu
. on cpu plug, re-add the cpu to all recorded cgroups for that
cpu (if any), as well as to any cgroups marked (in some /etc
file) as using "all" or a percentage of all cpus.

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