Message-ID: <65ad7f42-c3a0-c587-c1c0-d76dda3bc455@amd.com>
Date: Fri, 4 Oct 2024 19:23:54 -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,
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 jmattson@...gle.com, leitao@...ian.org, jpoimboe@...nel.org,
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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 4:09 PM, Reinette Chatre wrote:
> Hi Babu,
> 
> On 10/4/24 12:36 PM, Moger, Babu wrote:
>> On 10/4/2024 11:52 AM, Reinette Chatre wrote:
>>> On 10/4/24 9:38 AM, Moger, Babu wrote:
>>>> On 10/3/2024 9:17 PM, Reinette Chatre wrote:
>>>>> On 10/3/24 6:11 PM, Moger, Babu wrote:
>>>>>> On 10/2/2024 1:19 PM, Reinette Chatre wrote:
>>>>>>> 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
> 
> I doubt that the implementation would be complex but it may take some effort for it
> to be efficient ... taking actions that involve changing kernel and HW state for each
> flag as it is encountered vs. parsing all flags and changing kernel and HW state once.
> 
> The risk is that a simple request like "domain=lt" may take twice as long when
> doing sequential processing. When users provide flags like "domain=_lt" to take advantage
> of sequential processing then there may be an argument like "user gets what is being asked
> for" when things are slower, but I am not sure the same can be true for a user
> that just wants to run "domain=lt".
> 
> To me it seems simpler to require that "_" always appears by itself and that

Ok. Lets go with this approach treating "_" as a special and cannot be 
combined with other flags. Seems simple to implement.

> any flags set by the user using "=" are combined during parsing so that they can be
> acted on in a single flow. If indeed users want to do something sequentially
> like "unassign all flags and then assign local MBM" then instead of "domain=_l"
> I think "domain=_;domain=l" could be used?

Yes. It can be done.
thanks
Babu Moger

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