linux-kernel - Re: [PATCH] cpusets: Make cpus_allowed and mems

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Message-ID: <5527DAAA.7030308@linux.vnet.ibm.com>
Date:	Fri, 10 Apr 2015 19:44:02 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>, Tejun Heo <tj@...nel.org>
CC:	Peter Zijlstra <peterz@...radead.org>, lizefan@...wei.com,
	anton@...ba.org, svaidy@...ux.vnet.ibm.com, rjw@...ysocki.net,
	paulmck@...ux.vnet.ibm.com, mingo@...nel.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	bharata@...ux.vnet.ibm.com
Subject: Re: [PATCH] cpusets: Make cpus_allowed and mems_allowed masks hotplug
 invariant

Hi Serge,

On 04/10/2015 02:43 AM, Serge E. Hallyn wrote:
> On Mon, Apr 06, 2015 at 01:47:35PM -0400, Tejun Heo wrote:
>> Hello, Preeti.
>>
>> On Thu, Apr 02, 2015 at 12:26:32PM +0530, Preeti U Murthy wrote:
>>> By ensuring that the user configured cpusets are untouched, I don't see
>>> how we affect userspace adversely. The expectation usually is that the
>>> kernel keeps track of the user configurations. If anything we would be
>>> fixing an undesired behavior, wouldn't we?
>>
>> The problem is not really about which behavior is "righter" but rather
>> it's fairly likely that there are users / tools out there expecting
>> the current behavior and they wouldn't be too happy to see the
>> behavior flipping underneath them.
>>
>> One way forward would be implementing a knob in cpuset which makes it
>> switch sbetween the old and new behaviors in the legacy hierarchy.
>> It's yucky but doable if absoluately necessary, but what's the reason
>> for you not being able to transition to the unified hierarchy (except
> 
> If the userspace is entirely new then this should work.  The
> unified hierarchy's behavior is not backward-compatible so any old
> software which tried to create cgroups (libcgroup, lxc, etc) will
> not work with it (since it won't, for instance, know to fill in 
> the enabled controllers in every newly created cgroup).
> 
> Preeti, can you confirm that you don't have any need to run any
> legacy programs which use cgroups?  Long as that's the case, new

I don't think I can vouch for this safely. I have posted out a V2 of
this patch adhering to Tejun's first suggestion. IMO that seemed like
a better option.

Regards
Preeti U Murthy

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