linux-kernel - Re: [RFC PATCH v3 03/17] x86/resctrl: Detect Assignable Bandwidth Monitoring feature details

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Date: Fri, 10 May 2024 11:34:12 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....com>, <corbet@....net>, <fenghua.yu@...el.com>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>
CC: <x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>,
	<rdunlap@...radead.org>, <tj@...nel.org>, <peterz@...radead.org>,
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	<jpoimboe@...nel.org>, <rick.p.edgecombe@...el.com>,
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	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<eranian@...gle.com>, <james.morse@....com>
Subject: Re: [RFC PATCH v3 03/17] x86/resctrl: Detect Assignable Bandwidth
 Monitoring feature details

Hi Babu,

On 5/10/2024 10:01 AM, Moger, Babu wrote:
> On 5/9/2024 10:18 PM, Reinette Chatre wrote:
>> On 5/9/2024 3:34 PM, Moger, Babu wrote:
>>> On 5/7/24 15:27, Reinette Chatre wrote:
>>>> On 5/6/2024 12:09 PM, Moger, Babu wrote:
>>>>> On 5/3/24 18:26, Reinette Chatre wrote:
>>>>>> On 3/28/2024 6:06 PM, Babu Moger wrote:
>>>>
>>>> ...
>>>>
>>>>>>> + * @mbm_assign_cntrs:    Maximum number of assignable counters
>>>>>>>    */
>>>>>>>   struct rdt_resource {
>>>>>>>       int            rid;
>>>>>>> @@ -188,6 +198,8 @@ struct rdt_resource {
>>>>>>>       struct list_head    evt_list;
>>>>>>>       unsigned long        fflags;
>>>>>>>       bool            cdp_capable;
>>>>>>> +    bool            mbm_assign_capable;
>>>>>>> +    u32                     mbm_assign_cntrs;
>>>>>>>   };
>>>>>>
>>>>>> Please check tabs vs spaces (in this whole series please).
>>>>>
>>>>> Sure. Will do.
>>>>>
>>>>>>
>>>>>> I'm thinking that a new "MBM specific" struct within
>>>>>> struct rdt_resource will be helpful to clearly separate the MBM related
>>>>>> data. This will be similar to struct resctrl_cache for
>>>>>> cache allocation and struct resctrl_membw for memory bandwidth
>>>>>> allocation.
>>>>>
>>>>> Did you mean to move all the fields for monitoring to move to new struct?
>>>>>
>>>>> Struct resctrl_mbm {
>>>>>            int                     num_rmid;
>>>>>            bool                    mbm_assign_capable;
>>>>>            u32                     mbm_assign_cntrs;
>>>>> }:
>>>>>
>>>>
>>>> Indeed, so not actually MBM specific but monitoring specific as you state (with
>>>> appropriate naming?). This is purely to help organize data within struct rdt_resource
>>>> and (similar to struct resctrl_cache and struct resctrl_membw) not a new
>>>> structure expected to be passed around. I think evt_list can also be a member.
>>>
>>> How about this?
>>>
>>> Lets keep assign_capable in main structure(like we have mon_capable) and
>>> move rest of them into new structure.
>>>
>>> Struct resctrl_mon {
>>>             int                     num_rmid;
>>>        struct list_head        evt_list;
>>>             u32                     num_assign_cntrs;
>>> }:
>>
>> This looks like a good start. It certainly supports ABMC. I do not yet
>> have a clear understanding about how this will support MPAM and soft-RMID/ABMC
>> since the current implementation has an implicit scope of one counter per event per
>> monitor group. It thus seem reasonable to have a new generic name of "cntrs".
> 
> How about renaming it to "assignable_counters"?
> 

As I mentioned in [1] the "assign" concept is not clear (just to me perhaps?). It
really seems to be the marketing name percolating into the implementation. Why is
"assignable" needed to be in a "counter" variable name? Is a variable not by definition
"assignable"? Why not just, for example, "num_cntrs"? I believe that things to be
as simple and obvious as possible ... this just helps to reduce confusion.

My previous comment regarding counter scope is not addressed by a rename though and
I do not expect you to have the answer but I would like us to keep in mind how these
"counters" could end up getting used. We may just later, when soft-RMID/ABMC and MPAM
is understood, need to add a "counter scope" as well.

Reinette

[1] https://lore.kernel.org/lkml/0d94849c-828c-4f10-a6f8-e26cc4554909@intel.com/