linux-kernel - Re: [PATCH v5 12/12] Documentation/x86: Update resctrl

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Date:   Mon, 3 Oct 2022 08:36:39 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     <babu.moger@....com>, <corbet@....net>, <tglx@...utronix.de>,
        <mingo@...hat.com>, <bp@...en8.de>
CC:     <fenghua.yu@...el.com>, <dave.hansen@...ux.intel.com>,
        <x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>,
        <akpm@...ux-foundation.org>, <quic_neeraju@...cinc.com>,
        <rdunlap@...radead.org>, <damien.lemoal@...nsource.wdc.com>,
        <songmuchun@...edance.com>, <peterz@...radead.org>,
        <jpoimboe@...nel.org>, <pbonzini@...hat.com>,
        <chang.seok.bae@...el.com>, <pawan.kumar.gupta@...ux.intel.com>,
        <jmattson@...gle.com>, <daniel.sneddon@...ux.intel.com>,
        <sandipan.das@....com>, <tony.luck@...el.com>,
        <james.morse@....com>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <bagasdotme@...il.com>,
        <eranian@...gle.com>
Subject: Re: [PATCH v5 12/12] Documentation/x86: Update resctrl_ui.rst for new
 features

Hi Babu,

On 10/3/2022 7:28 AM, Moger, Babu wrote:
> Hi Reinette,
> 
> On 9/29/22 17:10, Reinette Chatre wrote:
>> Hi Babu,
>>
>> In subject: resctrl_ui.rst -> resctrl.rst
>>
>> On 9/27/2022 1:27 PM, Babu Moger wrote:
>>> Update the documentation for the new features:
>>> 1. Slow Memory Bandwidth allocation (SMBA).
>>>    With this feature, the QOS  enforcement policies can be applied
>>>    to the external slow memory connected to the host. QOS enforcement
>>>    is accomplished by assigning a Class Of Service (COS) to a processor
>>>    and specifying allocations or limits for that COS for each resource
>>>    to be allocated.
>>>
>>> 2. Bandwidth Monitoring Event Configuration (BMEC).
>>>    The bandwidth monitoring events mbm_total_bytes and mbm_local_bytes
>>>    are set to count all the total and local reads/writes respectively.
>>>    With the introduction of slow memory, the two counters are not
>>>    enough to count all the different types are memory events. With the
>> types are memory events -> types of memory events?
> Ok Sure
>>
>>>    feature BMEC, the users have the option to configure mbm_total_bytes
>>>    and mbm_local_bytes to count the specific type of events.
>>>
>>> Also add configuration instructions with examples.
>>>
>>> Signed-off-by: Babu Moger <babu.moger@....com>
>>> ---
>> ...
>>
>>> +
>>> +"mbm_total_config", "mbm_local_config":
>>> +        These files contain the current event configuration for the events
>>> +        mbm_total_bytes and mbm_local_bytes, respectively, when the
>>> +        Bandwidth Monitoring Event Configuration (BMEC) feature is supported.
>>> +        The event configuration settings are domain specific. Changing the
>>> +        configuration on one CPU in a domain would affect the whole domain.
>> This contradicts the implementation done in this series where the
>> configuration is changed on every CPU in the domain.
> 
> How about this?
> 
> The event configuration settings are domain specific and will affect all the CPUs in the domain.

There remains a disconnect between this and the implementation that writes the
configuration to every CPU.

You could make this change to the documentation but then the
implementation needs more than "Update MSR_IA32_EVT_CFG_BASE MSR on all
the CPUs in cpu_mask" - that comment needs to highlight that the
implementation does not follow the architecture and scope rules nor how
configuration changes are made in the rest of the driver and why. Previously [1]
you indicated that this is based on guidance from hardware team so perhaps you
could document it as a hardware quirk related to this feature? At the minimum
it should acknowledge the disconnect.

Reinette

[1] https://lore.kernel.org/lkml/3511f4f6-d043-9a22-7779-af2c2983b6a2@amd.com/