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

Waiman Long
2018-05-01T20:33:50Z

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@redhat.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