linux-kernel - Re: [PATCH v7 4/5] cpuset: Restrict load balancing off cpus to subset of cpus.isolated

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Date:   Tue, 1 May 2018 16:33:45 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Peter Zijlstra <peterz@...radead.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 4/5] cpuset: Restrict load balancing off cpus to subset
 of cpus.isolated

On 05/01/2018 03:51 PM, Tejun Heo wrote:
> Hello, Waiman.
>
> Sorry about the delay.
>
> On Thu, Apr 19, 2018 at 09:47:03AM -0400, Waiman Long wrote:
>> With the addition of "cpuset.cpus.isolated", it makes sense to add the
>> restriction that load balancing can only be turned off if the CPUs in
>> the isolated cpuset are subset of "cpuset.cpus.isolated".
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>  Documentation/cgroup-v2.txt |  7 ++++---
>>  kernel/cgroup/cpuset.c      | 29 ++++++++++++++++++++++++++---
>>  2 files changed, 30 insertions(+), 6 deletions(-)
>>
>> diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
>> index 8d89dc2..c4227ee 100644
>> --- a/Documentation/cgroup-v2.txt
>> +++ b/Documentation/cgroup-v2.txt
>> @@ -1554,9 +1554,10 @@ Cpuset Interface Files
>>  	and will not be moved to other CPUs.
>>  
>>  	This flag is hierarchical and is inherited by child cpusets. It
>> -	can be turned off only when the CPUs in this cpuset aren't
>> -	listed in the cpuset.cpus of other sibling cgroups, and all
>> -	the child cpusets, if present, have this flag turned off.
>> +	can be explicitly turned off only when it is a direct child of
>> +	the root cgroup and the CPUs in this cpuset are subset of the
>> +	root's "cpuset.cpus.isolated".	Moreover, the CPUs cannot be
>> +	listed in the "cpuset.cpus" of other sibling cgroups.
> It is a little bit convoluted that the isolation requires coordination
> among root's isolated file and the first-level children's cpus file
> and the flag.  Maybe I'm missing something but can't we do something
> like the following?
>
> * Add isolated flag file, which can only be modified on empty (in
>   terms of cpus) first level children.
>
> * Once isolated flag is set, CPUs can only be added to the cpus file
>   iff they aren't being used by anyone else and automatically become
>   isolated.
>
> The first level cpus file is owned by the root cgroup anyway, so
> there's no danger regarding delegation or whatever and the interface
> would be a lot simpler.

I think that will work too. We currently don't have a flag to make a
file visible on first-level children only, but it shouldn't be hard to
make one.

Putting CPUs into an isolated child cpuset means removing it from the
root's effective CPUs. So I would probably like to expose the read-only
cpus.effective in the root cgroup so that we can check changes in the
effective cpu list.

I will renew the patchset will your suggestion.

Thanks,
Longman