Message-ID: <1f7addbf-51e3-7f24-ad64-b8a6d9c5e0ba@amd.com>
Date: Fri, 4 Oct 2024 14:36:46 -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,
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 maciej.wieczor-retman@...el.com, linux-doc@...r.kernel.org,
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Subject: Re: [PATCH v7 24/24] x86/resctrl: Introduce interface to modify
 assignment states of the groups

Hi Reinette,

On 10/4/2024 11:52 AM, Reinette Chatre wrote:
> Hi Babu,
> 
> On 10/4/24 9:38 AM, Moger, Babu wrote:
>> Hi Reinette,
>>
>> On 10/3/2024 9:17 PM, Reinette Chatre wrote:
>>> Hi Babu,
>>>
>>> On 10/3/24 6:11 PM, Moger, Babu wrote:
>>>> Hi Reinette,
>>>>
>>>> On 10/2/2024 1:19 PM, Reinette Chatre wrote:
>>>>> Hi Babu,
>>>>>
>>>>> On 9/27/24 10:47 AM, Moger, Babu wrote:
>>>>>> On 9/19/2024 12:59 PM, Reinette Chatre wrote:
>>>>>>> On 9/4/24 3:21 PM, Babu Moger wrote:
>>>>>
>>>>>>>> v7: Simplified the parsing (strsep(&token, "//") in rdtgroup_mbm_assign_control_write().
>>>>>>>>         Added mutex lock in rdtgroup_mbm_assign_control_write() while processing.
>>>>>>>>         Renamed rdtgroup_find_grp to rdtgroup_find_grp_by_name.
>>>>>>>>         Fixed rdtgroup_str_to_mon_state to return error for invalid flags.
>>>>>>>>         Simplified the calls rdtgroup_assign_cntr by merging few functions earlier.
>>>>>>>>         Removed ABMC reference in FS code.
>>>>>>>>         Reinette commented about handling the combination of flags like 'lt_' and '_lt'.
>>>>>>>>         Not sure if we need to change the behaviour here. Processed them sequencially right now.
>>>>>>>>         Users have the liberty to pass the flags. Restricting it might be a problem later.
>>>>>>>
>>>>>>> Could you please give an example of what problem may be encountered later? An assignment
>>>>>>> like "domain=_lt" seems like a contradiction to me since user space essentially asks
>>>>>>> for "None of the MBM events" as well as "MBM total event" and "MBM local event".
>>>>>>
>>>>>> I agree it is contradiction. But user is the one who decides to do that. I think we should allow it. Also, there is some value to it as well.
>>>>>>
>>>>>> "domain=_lt" This will also reset the counters if the total and local events are assigned earlier this action.
>>>>>
>>>>> The last sentence is not clear to me. Could you please elaborate what
>>>>> you mean with "are assigned earlier this action"?
>>>>>
>>>>
>>>> I think I confused you here. "domain=_lt" is equivalent to "domain=lt".  My reasoning is handling all the combination in the code adds code complexity and leave it the user what he wants to do.
>>>
>>> hmmm ... and how about "domain=lt_"? Do you think this should also be equivalent to
>>> "domain=lt" or perhaps an expectation that counters should be assigned to the two events
>>> and then immediately unassigned?
>>
>> Yes. "domain=lt_" should be "domain=lt".
>>
>>>
>>> Giving user such flexibility may be interpreted as the assignment seen as acting
>>> sequentially through the flags provided. Ideally the interface should behave in
>>> a predictable way if the goal is to provide flexibility to the user.
>>>
>>
>> My only concern is adding the check now and reverting it back later.
>> Basically process the flags sequentially and don't differentiate between the flags. I feel it fits the predictable behavior. No?
> 
> This is the point I was trying to make. If flags are processed sequentially then it would be
> predictable behavior and if that is documented expectation then that should be ok. The problem
> that I want to highlight is that if predictable sequential processing is the goal then
> "domain=_lt" cannot be interpreted the same as "domain="lt_". When flags in "domain=lt_"
> are processed sequentially then final state should be "domain=_", no?

Yes. that is correct.
> 
> If sequential processing is done then "domain=_lt" means "unassign all counters followed
> by assign of counter to local MBM monitoring, followed by assign of counter to total MBM
> monitoring". Similarly, "domain=lt_" means "assign a counter to local MBM monitoring, then
> assign a counter to total MBM monitoring, then unassign all counters".

Yes. That is correct.
> 
> If this sequential processing is the goal then the implementation would still need to be
> adapted. Consider, for example, "domain=lt" ... with sequential processing the user
> indicates/expects that "local MBM monitoring" has priority if there is only one counter
> available, but the current implementation does not process it sequentially and would end up
> assigning counter to "total MBM monitoring" first.

Sure. Lets accommodate the sequential processing. Process the  flags in 
the order it is provided. I need to make few changes to 
rdtgroup_process_flags() to address it. Hopefully, it can be done 
without much complexity. Thanks
-- 
- Babu Moger

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