linux-kernel - Re: [PATCH v9 19/26] x86/resctrl: Add the interface to unassign a MBM counter

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Message-ID: <6fd12815-98eb-469f-8477-0de12af29aa2@amd.com>
Date: Mon, 4 Nov 2024 12:21:25 -0600
From: "Moger, Babu" <babu.moger@....com>
To: Peter Newman <peternewman@...gle.com>
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Subject: Re: [PATCH v9 19/26] x86/resctrl: Add the interface to unassign a MBM
 counter

Hi Peter,

On 11/4/24 08:16, Peter Newman wrote:
> Hi Babu,
> 
> On Wed, Oct 30, 2024 at 12:25 AM Babu Moger <babu.moger@....com> wrote:
>>
>> The mbm_cntr_assign mode provides a limited number of hardware counters
>> that can be assigned to an RMID, event pair to monitor bandwidth while
>> assigned. If all counters are in use, the kernel will show an error
>> message: "Out of MBM assignable counters" when a new assignment is
>> requested. To make space for a new assignment, users must unassign an
>> already assigned counter.
>>
>> Introduce an interface that allows for the unassignment of counter IDs
>> from both the group and the domain. Additionally, ensure that the global
>> counter is released if it is no longer assigned to any domains.
> 
> This seems unnecessarily restrictive. What's wrong with monitoring
> different groups in different domains?

Yes. User can monitor different groups in different domains. But, they
will have to use different global counter for each group.

Here is an example.

#cat /sys/fs/resctrl/info/L3_MON/mbm_assign_control
groupA//0=t;1=_;
groupB//0=_;1=l;

Group A - counter 0 (Assigned to total event in Domain 0)
Group B - counter 1 (Assigned to local event in Domain 1)

We allocate two different counters here.  Now we are left with 30 counters
(max 32).


This is similar to CLOSID management we follow in resctrl. This is not a
new restriction,
-- 
Thanks
Babu Moger