linux-kernel - Re: [PATCH v11 10/23] x86/resctrl: Remove MSR reading of event configuration value

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Message-ID: <2a78f9a1-e03a-4e82-836b-26d3175d3f2b@intel.com>
Date: Wed, 5 Feb 2025 17:41:53 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: "Luck, Tony" <tony.luck@...el.com>, Babu Moger <babu.moger@....com>,
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 Stephane" <eranian@...gle.com>
Subject: Re: [PATCH v11 10/23] x86/resctrl: Remove MSR reading of event
 configuration value

Hi Tony,

On 2/5/25 4:51 PM, Luck, Tony wrote:
>> This new arch API has sharp corners because of asymmetry of where resctrl
>> runs the arch function. I do not think it is required to change this since we
>> can only speculate about how this may be used in the future but I do think
>> it will be helpful to add comments that highlight:
>>
>> resctrl_arch_mon_event_config_get() ->  May run on CPU that does not belong to domain.
>> resctrl_arch_mon_event_config_set() ->  Runs on CPU that belongs to domain.
> 
> Here's a vague data point about the future to help with speculation.
> 
> I have something coming along the pipeline that also can run on any CPU.
> 
> I am contemplating a flag in the rdt_resource structure (in appropriate substructure
> resctrl_cache/resctrl_membw) to indicate "domain" vs. "any" for operations.
> 
> Would something like that be useful here?

hmm ... I cannot envision how this may look. Could you please elaborate?

You mention "a" (singular) flag in rdt_resource while this scenario involves
different ops having different scope. This makes me think that this flag may
have to be per operation that in turn would need additional infrastructure to
manage and track operations.

These "arch" functions are evolving as the work to support MPAM is done and
so far I think it has been quite ad-hoc to just refactor arch specific code
into "arch" helpers instead of keeping track of which scope they are running in.
This currently requires any arch implementing an "arch" helper to be well aware 
of how resctrl will call it.

Reinette