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

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Date:   Wed, 26 Oct 2022 11:36:48 +0200
From:   Peter Newman <peternewman@...gle.com>
To:     James Morse <james.morse@....com>
Cc:     Reinette Chatre <reinette.chatre@...el.com>,
        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 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.
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.

> 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?

-Peter