Re: [PATCH v2 2/6] cgroup/cpuset: Clarify the use of invalid partition root

From: Waiman Long
Date: Fri Jul 16 2021 - 14:59:52 EST

On 7/16/21 2:44 PM, Waiman Long wrote:
On 7/5/21 1:51 PM, Tejun Heo wrote:
Hello, Waiman.

On Mon, Jun 28, 2021 at 09:06:50AM -0400, Waiman Long wrote:
The main reason for doing this is because normal cpuset control file actions
are under the direct control of the cpuset code. So it is up to us to decide
whether to grant it or deny it. Hotplug, on the other hand, is not under the
control of cpuset code. It can't deny a hotplug operation. This is the main
reason why the partition root error state was added in the first place.
I have a difficult time convincing myself that this difference justifies the
behavior difference and it keeps bothering me that there is a state which
can be reached through one path but rejected by the other. I'll continue
below.

Normally, users can set cpuset.cpus to whatever value they want even though
they are not actually granted. However, turning on partition root is under
more strict control. You can't turn on partition root if the CPUs requested
cannot actually be granted. The problem with setting the state to just
partition error is that users may not be aware that the partition creation
operation fails.  We can't assume all users will do the proper error
checking. I would rather let them know the operation fails rather than
relying on them doing the proper check afterward.

Yes, I agree that it is a different philosophy than the original cpuset
code, but I thought one reason of doing cgroup v2 is to simplify the
interface and make it a bit more erorr-proof. Since partition root creation
is a relatively rare operation, we can afford to make it more strict than
the other operations.
So, IMO, one of the reasons why cgroup1 interface was such a mess was
because each piece of interaction was designed ad-hoc without regard to the
overall consistency. One person feels a particular way of interacting with
the interface is "correct" and does it that way and another person does
another part in a different way. In the end, we ended up with a messy
patchwork.

One problematic aspect of cpuset in cgroup1 was the handling of failure
modes, which was caused by the same exact approach - we wanted the interface
to reject invalid configurations outright even though we didn't have the
ability to prevent those configurations from occurring through other paths,
which makes the failure mode more subtle by further obscuring them.

I think a better approach would be having a clear signal and mechanism to
watch the state and explicitly requiring users to verify and monitor the
state transitions.

Sorry for the late reply as I was busy with other works.

I agree with you on principle. However, the reason why there are more restrictions on enabling partition is because I want to avoid forcing the users to always read back cpuset.partition.type to see if the operation succeeds instead of just getting an error from the operation. The former approach is more error prone. If you don't want changes in existing behavior, I can relax the checking and allow them to become an invalid partition if an illegal operation happens.

Also there is now another cpuset patch to extend cpu isolation to cgroup v1 [1]. I think it is better suit to the cgroup v2 partition scheme, but cgroup v1 is still quite heavily out there.

Please let me know what you want me to do and I will send out a v3 version.

Note that the current cpuset partition implementation have implemented some restrictions on when a partition can be enabled. However, I missed some corner cases in the original implementation that allow certain cpuset operations to make a partition invalid. I tried to plug those holes in this patchset. However, if maintaining backward compatibility is more important, I can leave those holes and update the documentation to make sure that people check cpuset.partition.type to confirm if their operation succeeds.

Cheers,
Longman