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

[Yu, Fenghua]

Hi, Peter,

> On 10/7/2022 3:39 AM, Peter Newman 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.

Are the "all tasks" children of the container process? Is it possible to move the
parent container process and its all children tasks to a different group in one shot
instead of one by one?

> >
> > 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.
> >
> > 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.

-Fenghua