linux-kernel - Re: RE: [PATCH v10 16/24] x86/resctrl: Add interface to the assign counter

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Message-ID: <daa28f1c-1064-4adc-a99c-fdad1bc08a9f@amd.com>
Date: Thu, 19 Dec 2024 16:33:59 -0600
From: "Moger, Babu" <bmoger@....com>
To: "Luck, Tony" <tony.luck@...el.com>,
 "Chatre, Reinette" <reinette.chatre@...el.com>,
 Babu Moger <babu.moger@....com>
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Subject: Re: RE: [PATCH v10 16/24] x86/resctrl: Add interface to the assign
 counter

Hi Tony,

On 12/19/2024 3:45 PM, Luck, Tony wrote:
>>>>>>>> It is right thing to continue assignment if one of the domain is out of
>>>>>>>> counters. In that case how about we save the error(say error_domain) and
>>>>>>>> continue. And finally return success if both ret and error_domain are zeros.
>>>>>>>>
>>>>>>>>        return ret ?  ret : error_domain:
>>>>>>>
>>>>>>> If there are many domains, then you might have 3 succeed and 5 fail.
>>>>>>>
>>>>>>> I think the best you can do is return success if everything succeeded
>>>>>>> and an error if any failed.
>>>>>>
>>>>>> Yes. The above check should take care of this case.
>>>>>>
>>>>>
>>>>> If I understand correctly "error_domain" can capture the ID of
>>>>> a single failing domain. If there are multiple failing domains like
>>>>> in Tony's example then "error_domain" will not be accurate and thus
>>>>> can never be trusted. Instead of a single check of a failure user
>>>>> space is then forced to parse the more complex "mbm_assign_control"
>>>>> file to learn what succeeded and failed.
>>>>>
>>>>> Would it not be simpler to process sequentially and then fail on
>>>>> first error encountered with detailed error message? With that
>>>>> user space can determine exactly which portion of request
>>>>> succeeded and which portion failed.
>>>>
>>>> One more option is to print the error for each failure and continue. And finally return error.
> 
> There's limited space allocated for use by last_cmd_*() messages:
> 
> static char last_cmd_status_buf[512];
> 
>          seq_buf_init(&last_cmd_status, last_cmd_status_buf,
>                       sizeof(last_cmd_status_buf));
> 
> If you keep parsing and trying to apply changes from user input you will
> quickly hit that limit.

oh. ok. Good to know.
> 
> 
>>>>
>>>> "Group mon1, domain:1 Out of MBM counters"
>>>>
>>>> We have the error information as well as the convenience of assignment on domains where counters are available when user is working with "*"(all domains).
>>>
>>> This may be possible. Please keep in mind that any errors have to be
>>> easily consumed in an automated way to support the user space tools
>>> that interact with resctrl. I do not think we have thus far focused
>>> on the "last_cmd_status" buffer as part of the user space ABI so this opens
>>> up more considerations.
>>>
>>> At this time the error handling of "all domains" does not seem to be
>>> consistent and obvious to user space. From what I can tell the
>>> implementation continues on to the next domain if one domain is out
>>> of counters but it exits immediately if a counter cannot be configured
>>> on a particular domain.
>>
>> Yes. We can handle both the errors in the same way.
> 
> I think it is simplest to make the "same way" be "fail on first error".

Ok. Sure.  Will do thanks.

-Babu