linux-kernel - Re: [PATCH v7 3/5] cpuset: Add a root-only cpus.isolated v2 control file

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Date:   Mon, 7 May 2018 20:30:02 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...com, pjt@...gle.com, luto@...capital.net,
        Mike Galbraith <efault@....de>, torvalds@...ux-foundation.org,
        Roman Gushchin <guro@...com>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v7 3/5] cpuset: Add a root-only cpus.isolated v2 control
 file

On 05/02/2018 10:08 AM, Peter Zijlstra wrote:
> On Thu, Apr 19, 2018 at 09:47:02AM -0400, Waiman Long wrote:
>> diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
>> index c970bd7..8d89dc2 100644
>> --- a/Documentation/cgroup-v2.txt
>> +++ b/Documentation/cgroup-v2.txt
>> @@ -1484,6 +1484,31 @@ Cpuset Interface Files
>>  	a subset of "cpuset.cpus".  Its value will be affected by CPU
>>  	hotplug events.
>>  
>> +  cpuset.cpus.isolated
>> +	A read-write multiple values file which exists on root cgroup
>> +	only.
>> +
>> +	It lists the CPUs that have been withdrawn from the root cgroup
>> +	for load balancing.  These CPUs can still be allocated to child
>> +	cpusets with load balancing enabled, if necessary.
>> +
>> +	If a child cpuset contains only an exclusive set of CPUs that are
>> +	a subset of the isolated CPUs and with load balancing enabled,
>> +	these CPUs will be load balanced on a separate root domain from
>> +	the one in the root cgroup.
>> +
>> +	Just putting the CPUs into "cpuset.cpus.isolated" will be
>> +	enough to disable load balancing on those CPUs as long as they
>> +	do not appear in a child cpuset with load balancing enabled.
>> +	Fine-grained control of cpu isolation can also be done by
>> +	putting these isolated CPUs into child cpusets with load
>> +	balancing disabled.
>> +
>> +	The "cpuset.cpus.isolated" should be set up before child
>> +	cpusets are created.  Once child cpusets are present, changes
>> +	to "cpuset.cpus.isolated" will not be allowed if the CPUs that
>> +	change their states are in any of the child cpusets.
>> +
> So I see why you did this, but it is _really_ ugly and breaks the
> container invariant.
>
> Ideally we'd make the root group less special, not more special.

Yes, I am planning to make the root cgroup less special by putting a new
isolation flag into all the non-root cgroup.

The container invariant thing, however, is a bit hard to do. Do we
really need a container root to behave exactly like the real root? I
guess we can make that happen if we really want to, but it will
certainly make the code more complex. So it is a trade-off about what is
worth to do and what is not.

Cheers,
Longman