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 20:40:32 -0500
From: "Moger, Babu" <bmoger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, 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, yanjiewtw@...il.com,
 kim.phillips@....com, lukas.bulwahn@...il.com, seanjc@...gle.com,
 jmattson@...gle.com, leitao@...ian.org, jpoimboe@...nel.org,
 rick.p.edgecombe@...el.com, kirill.shutemov@...ux.intel.com,
 jithu.joseph@...el.com, kai.huang@...el.com, kan.liang@...ux.intel.com,
 daniel.sneddon@...ux.intel.com, pbonzini@...hat.com, sandipan.das@....com,
 ilpo.jarvinen@...ux.intel.com, peternewman@...gle.com,
 maciej.wieczor-retman@...el.com, 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 Reinette,


On 5/10/2024 1:34 PM, Reinette Chatre wrote:
> 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.

Ok. That sounds fine to me.
> 
> 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.

Sure.

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

Thanks
- Babu Moger