linux-kernel - Re: [PATCH v3 06/19] x86/resctrl: Allow the allocator to check if a CLOSID can allocate clean RMID

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Message-ID: <37c2371c-11e6-a2ef-059a-1a0ba08c2f67@intel.com>
Date:   Fri, 31 Mar 2023 16:21:12 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     James Morse <james.morse@....com>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>
CC:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        <shameerali.kolothum.thodi@...wei.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        <carl@...amperecomputing.com>, <lcherian@...vell.com>,
        <bobo.shaobowang@...wei.com>, <tan.shaopeng@...itsu.com>,
        <xingxin.hx@...nanolis.org>, <baolin.wang@...ux.alibaba.com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Xin Hao <xhao@...ux.alibaba.com>, <peternewman@...gle.com>
Subject: Re: [PATCH v3 06/19] x86/resctrl: Allow the allocator to check if a
 CLOSID can allocate clean RMID

Hi James,

On 3/20/2023 10:26 AM, James Morse wrote:

...

> +/**
> + * resctrl_closid_is_dirty - Determine if all RMID associated with this CLOSID
> + *                           are available.
> + * @closid: The CLOSID that is being queried.
> + *
> + * MPAM's equivalent of RMID are per-CLOSID, meaning a freshly allocated CLOSID
> + * may not be able to allocate clean RMID. To avoid this the allocator will
> + * only return clean CLOSID. This is enough for now as it allows MPAM systems
> + * to use resctrl. This suffers from the problem that there may be no CLOSID
> + * where all the RMID are clean, causing the CLOSID allocation to fail.
> + * This can be improved (once MPAM support is upstream) to return the cleanest
> + * CLOSID where PMG=0 is clean. This would allow the CLOSID allocation to

Why does PMG=0 have to be the clean ID? 

I am wondering about the use cases here. When a new CLOSID needs to be allocated,
would it not be useful to instead have a utility that returns the "cleanest" CLOSID?
Instead of picking an available CLOSID and then always have to check if it is
"dirty or not", why not have a utility that picks the CLOSID with the most
available PMGs?

Reinette