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

[Stephane Eranian]

Hi,

On Fri, Oct 7, 2022 at 3:39 AM Peter Newman <peternewman@xxxxxxxxxx> wrote:
>
> Hi Reinette, Fenghua,
>
> I'd like to talk about the tasks file interface in CTRL_MON and MON
> groups.
>
> For some background, we are using the memory-bandwidth monitoring and
> allocation features of resctrl to maintain QoS on external memory
> bandwidth for latency-sensitive containers to help enable batch
> containers to use up leftover CPU/memory resources on a machine.  We
> also monitor the external memory bandwidth usage of all hosted
> containers to identify ones which are misusing their latency-sensitive
> CoS assignment and downgrade them to the batch CoS.
>
> The trouble is, container manager developers working with the tasks
> interface have complained that it's not usable for them because it takes
> many (or an unbounded number of) passes to move all tasks from a
> container over, as the list is always changing.
>
> Our solution for them is to remove the need for moving tasks between
> CTRL_MON groups. Because we are mainly using MB throttling to implement
> QoS, we only need two classes of service. Therefore we've modified
> resctrl to reuse existing CLOSIDs for CTRL_MON groups with identical
> configurations, allowing us to create a CTRL_MON group for every
> container. Instead of moving the tasks over, we only need to update
> their CTRL_MON group's schemata. Another benefit for us is that we do
> not need to also move all of the tasks over to a new monitoring group in
> the batch CTRL_MON group, and the usage counts remain intact.
>
> The CLOSID management rules would roughly be:
>
>  1. If an update would cause a CTRL_MON group's config to match that of
>     an existing group, the CTRL_MON group's CLOSID should change to that
>     of the existing group, where the definition of "match" is: all
>     control values match in all domains for all resources, as well as
>     the cpu masks matching.
>
>  2. If an update to a CTRL_MON group sharing a CLOSID with another group
>     causes that group to no longer match any others, a new CLOSID must
>     be allocated.
>
>  3. An update to a CTRL_MON group using a non-shared CLOSID which
>     continues to not match any others follows the current resctrl
>     behavior.
>
Another important aspect of this change is that unlike the default
model of moving all the threads to the
control group corresponding to the restriction, it allows each
container group (cgroup) to have its own
resctrl group and therefore its own RMID, and therefore its own
monitoring capabilities. This is important
when we need to track who is responsible for bandwidth consumption,
for instance.

>
> Before I prepare any patches for review, I'm interested in any comments
> or suggestions on the use case and solution.
>
> Are there simpler strategies for reassigning a running container's tasks
> to a different CTRL_MON group that we should be considering first?
>
> Any concerns about the CLOSID-reusing behavior? The hope is existing
> users who aren't creating identically-configured CTRL_MON groups would
> be minimally impacted. Would it help if the proposed behavior were
> opt-in at mount-time?
>
> Thanks!
> -Peter