linux-kernel - Re: [PATCH v8 19/25] x86/resctrl: Auto assign/unassign counters when mbm_cntr

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Message-ID: <42c4ac2f-2b18-48d9-89b4-5c88659bee2e@amd.com>
Date: Tue, 15 Oct 2024 15:42:46 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>,
 "Luck, Tony" <tony.luck@...el.com>
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Subject: Re: [PATCH v8 19/25] x86/resctrl: Auto assign/unassign counters when
 mbm_cntr_assign is enabled

Hi Reinette,

On 10/15/24 12:18,  wrote:
> Hi Babu,
> 
> On 10/15/24 8:43 AM, Moger, Babu wrote:
>> Hi Reinette/Tony,
>>
>> On 10/14/24 21:39,  wrote:
>>> Hi Babu,
>>>
>>> On 10/14/24 9:35 AM, Moger, Babu wrote:
>>>> On 12/31/69 18:00, Luck, Tony wrote:
>>>  
>>>>>
>>>>> It is still the case that callers don't care about the return value.
>>>>
>>>> That is correct.
>>>>
>>>
>>> Are you planning to change this? I think Tony has a good point that since
>>> assignment failures do not matter it unnecessarily complicates the code to
>>> have rdtgroup_assign_cntrs() return failure.
>>>
>>> I also think the internals of rdtgroup_assign_cntrs() deserve a closer look.
>>> I assume that error handling within rdtgroup_assign_cntrs() was created with
>>> ABMC in mind. When only considering ABMC then the only reason why
>>> rdtgroup_assign_cntr_event() could fail is if the system ran out of counters
>>> and then indeed it makes no sense to attempt another call to rdtgroup_assign_cntr_event().
>>>
>>> Now that the resctrl fs/arch split is clear the implementation does indeed expose
>>> another opportunity for failure ... if the arch callback, resctrl_arch_config_cntr()
>>> fails. It could thus be possible for the first rdtgroup_assign_cntr_event() to fail
>>> while the second succeeds. Earlier [1], Tony suggested to, within rdtgroup_assign_cntrs(),
>>> remove the local ret variable and have it return void. This sounds good to me.
>>> When doing so a function comment explaining the usage will be helpful.
>>>
>>> I also think that rdtgroup_unassign_cntrs() deserves similar scrutiny. Even more
>>> so since I do not think that the second rdtgroup_unassign_cntr_event()
>>> should be prevented from running if the first rdtgroup_unassign_cntr_event() fails.
>>
>>
>> Sounds fine with me. Now it will look like this below.
> 
> Thank you for considering.
> 
>>
>>
> 
> I assume that you will keep rdtgroup_assign_cntrs() function comment? I think
> it may need some small changes to go with the function now returning void ...
> for example, saying "Each group *requires* two counters" and then not failing when
> two counters cannot be allocated seems suspect.
> 
> For example (please feel free to improve):
> 
> 	Called when a new group is created. If "mbm_cntr_assign" mode is enabled,   
> 	counters are automatically assigned. Each group can accommodate two counters:      
> 	one for the total event and one for the local event. Assignments may fail
> 	due to the limited number of counters. However, it is not necessary to
> 	fail the group creation and thus no failure is returned. Users have the
> 	option to modify the counter assignments after the group has been created.   
> 

Looks good. Thanks

-- 
Thanks
Babu Moger