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

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Date:	Wed, 15 Apr 2015 11:36:17 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	svaidy@...ux.vnet.ibm.com, nacc@...ux.vnet.ibm.com,
	rjw@...ysocki.net, linux-kernel@...r.kernel.org,
	lizefan@...wei.com, anton@...ba.org, bharata@...ux.vnet.ibm.com,
	cgroups@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	mingo@...nel.org
Subject: Re: [PATCH V2] cpuset: Add knob to make allowed masks hotplug
 invariant on legacy hierarchy

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
there.

>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.

-serge
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