Re: [PATCH v11 17/23] x86/resctrl: Auto assign/unassign counters when mbm_cntr_assign is enabled

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Dave and Peter,

On 2/19/25 6:09 AM, Peter Newman wrote:
> Hi Dave,
> 
> On Wed, Feb 19, 2025 at 2:41 PM Dave Martin <Dave.Martin@arm.com> wrote:
>>
>> Hi,
>>
>> On Wed, Jan 22, 2025 at 02:20:25PM -0600, Babu Moger wrote:
>>> Assign/unassign counters on resctrl group creation/deletion. Two counters
>>> are required per group, one for MBM total event and one for MBM local
>>> event.
>>>
>>> There are a limited number of counters available for assignment. If these
>>> counters are exhausted, the kernel will display the error message: "Out of
>>> MBM assignable counters". However, it is not necessary to fail the
>>> creation of a group due to assignment failures. Users have the flexibility
>>> to modify the assignments at a later time.
>>
>> If we are doing this, should turning mbm_cntr_assign mode on also
>> trigger auto-assingment for all extant monitoring groups?
>>
>> Either way though, this auto-assignment feels like a potential nuisance
>> for userspace.

hmmm ... this auto-assignment was created with the goal to help userspace.
In mbm_cntr_assign mode the user will only see data when a counter is assigned
to an event. mbm_cntr_assign mode is selected as default on a system that
supports ABMC. Without auto assignment a user will thus see different
behavior when reading the monitoring events when the user switches to a kernel with
assignable counter support: Before assignable counter support events will have
data, with assignable counter support the events will not have data. 

I understood that interfaces should not behave differently when user space
switches kernels and that is what the auto assignment aims to solve.

>>
>> If the userspace use-case requires too many monitoring groups for the
>> available counters, then the kernel will auto-assign counters to a
>> random subset of groups which may or may not be the ones that userspace
>> wanted to monitor; then userspace must manually look for the assigned
>> counters and unassign some of them before they can be assigned where
>> userspace actually wanted them.
>>
>> This is not impossible for userspace to cope with, but it feels
>> awkward.
>>
>> Is there a way to inhibit auto-assignment?
>>
>> Or could automatic assignments be considered somehow "weak", so that
>> new explicit assignments can steal automatically assigned counters
>> without the need to unassign them explicitly?
> 
> We had an incomplete discussion about this early on[1]. I guess I
> didn't revisit it because I found it was trivial to add a flag that
> inhibits the assignment behavior during mkdir and had moved on to
> bigger issues.

Could you please remind me how a user will set this flag?

> 
> If an agent creating directories isn't coordinated with the agent
> managing counters, a series of creating and destroying a group could
> prevent a monitor assignment from ever succeeding because it's not
> possible to atomically discover the name of the new directory that
> stole the previously-available counter and reassign it.
> 
> However, if the counter-manager can get all the counters assigned once
> and only move them with atomic reassignments, it will become
> impossible to snatch them with a mkdir.
> 

You have many points that makes auto-assignment not be ideal but I
remain concerned that not doing something like this will break
existing users who are not as familiar with resctrl internals.

Reinette