Re: [RFD] resctrl: reassigning a running container's CTRL_MON group

[James Morse]

Hi Reinette,

On 12/10/2022 18:23, Reinette Chatre wrote:
> On 10/12/2022 4:21 AM, Peter Newman wrote:
>> On Tue, Oct 11, 2022 at 1:35 AM Reinette Chatre
>> <reinette.chatre@xxxxxxxxx> wrote:
>>>
>>> On 10/7/2022 10:28 AM, Tony Luck wrote:
>>>> I don't know how complex it would for the kernel to implement this. Or
>>>> whether it would meet Google's needs.
>>>>
>>>
>>> How about moving monitor groups from one control group to another?
>>>
>>> Based on the initial description I got the impression that there is
>>> already a monitor group for every container. (Please correct me if I am
>>> wrong). If this is the case then it may be possible to create an interface
>>> that could move an entire monitor group to another control group. This would
>>> keep the benefit of usage counts remaining intact, tasks get a new closid, but
>>> keep their rmid. There would be no need for the user to specify process-ids.
>>
>> Yes, Stephane also pointed out the importance of maintaining RMID assignments
>> as well and I don't believe I put enough emphasis on it during my
>> original email.
>>
>> We need to maintain accurate memory bandwidth usage counts on all
>> containers, so it's important to be able to maintain an RMID assignment
>> and its event counts across a CoS downgrade. The solutions Tony
>> suggested do solve the races in moving the tasks, but the container
>> would need to temporarily join the default MON group in the new CTRL_MON
>> group before it can be moved to its replacement MON group.
>>
>> Being able to re-parent a MON group would allow us to change the CLOSID
>> independently of the RMID in a container and would address the issue.

> What if resctrl adds support to rdtgroup_kf_syscall_ops for
> the .rename callback?
> 
> It seems like doing so could enable users to do something like:
> mv /sys/fs/resctrl/groupA/mon_groups/containerA /sys/fs/resctrl/groupB/mon_groups/
> 
> Such a user request would trigger the "containerA" monitor group
> to be moved to another control group. All tasks within it could be moved to
> the new control group (their CLOSIDs are changed) while their RMIDs
> remain intact.
> 
> I just read James's response and I do not know how this could be made to
> work with the Arm monitoring when it arrives. Potentially there
> could be an architecture specific "move monitor group" call.

If its just moving tasks between groups - this should be fine. You'll get some noise, but
this already exists. User-space should understand that what it is monitoring has changed
in this case.

My comments were about having the kernel transparently change the closid in response to a
schema change. This is where user-space can't know that it is now monitoring something
else. (maybe I should have replied to the top of the thread).



Thanks,

James