linux-kernel - Re: [RFD] resctrl: reassigning a running container's CTRL

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Date:   Tue, 8 Nov 2022 13:25:50 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     James Morse <james.morse@....com>,
        Peter Newman <peternewman@...gle.com>
CC:     Tony Luck <tony.luck@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Babu Moger" <Babu.Moger@....com>,
        Gaurang Upasani <gupasani@...gle.com>
Subject: Re: [RFD] resctrl: reassigning a running container's CTRL_MON group

Hi James,

On 11/3/2022 10:06 AM, James Morse wrote:
> Hi Peter,
> 
> On 26/10/2022 10:36, Peter Newman wrote:
>> On Tue, Oct 25, 2022 at 5:56 PM James Morse <james.morse@....com> wrote:
>>> This would work when systems are built to look like RDT, but MPAM has other control types
>>> where this would have interesting behaviours.
>>>
>>> 'CPOR' is equivalent to CBM as they are both a bitmap of portions. MPAM also has 'CMAX'
>>> where a fraction of the cache is specified. If you create two control groups with
>>> different PARTIDs but the same configuration, their two 50%s of the cache could become
>>> 100%. CPOR can be used like this, CMAX can't.
> 
>> I thought we only allocated caches with CBMs and memory bandwidth with
>> percentages.
> 
> Those are the existing schema, yes.
> 
> 
>> I don't see how CMAX could be used when implementing resctrl's CAT
>> resources. Percentage
>> configurations are only used for MBA in resctrl today.
> 
> The problem is if you say "CLOSID/PARTID are random, its the configuration that matters",
> you've broken all the control types where the regulation is happening based on the PARTID
> and the configuration, not the configuration alone.
> 
> If you do this, you can't ever have schema that use those configuration schemes.
> There is hardware out there that supports these schemes.
> 
> 
>>> Even when the controls behave in the same way, a different PARTID with the same control
>>> values could be regulated differently, resulting in weirdness.
>>
>> Can you provide further examples?
> 
> CMAX, MBW_MIN and MBW_MAX: You can have 50%, and I can have 50%. Your secret clones which
> have different PARTID and a copy of your configuration also get 50%. As far as the
> hardware is concerned, we're trying to play with more than 100% of the resource.
> 
> I don't know what the memory controller people are building, but naively I think the MBW
> MIN/MAX stuff is a more natural fit that a bandwidth bitmap.
> 
> 
> You couldn't ever add new configuration schemes that are based on a fraction or percentage
> of the resource.

Thank you very much for catching this early and highlighting this. Yes,
MBA also falls into this category so using different PARTID/CLOSID in the
same control group is not an option.

Reinette