linux-kernel - Re: [PATCH 06/18] x86/resctrl: Allow the allocator to check if a CLOSID can allocate clean RMID

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Message-ID: <bd31ed69-7748-b46c-f98a-3701e1de3e0d@arm.com>
Date:   Wed, 9 Nov 2022 17:02:40 +0000
From:   James Morse <james.morse@....com>
To:     Shawn Wang <shawnwang@...ux.alibaba.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...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,
        Jamie Iles <quic_jiles@...cinc.com>,
        Xin Hao <xhao@...ux.alibaba.com>, xingxin.hx@...nanolis.org,
        baolin.wang@...ux.alibaba.com, peternewman@...gle.com
Subject: Re: [PATCH 06/18] x86/resctrl: Allow the allocator to check if a
 CLOSID can allocate clean RMID

Hi Shawn,

On 08/11/2022 15:57, Shawn Wang wrote:
> On 10/21/2022 9:11 PM, James Morse wrote:
>> MPAM's PMG bits extend its PARTID space, meaning the same PMG value can be
>> used for different control groups.
>>
>> This means once a CLOSID is allocated, all its monitoring ids may still be
>> dirty, and held in limbo.
>>
>> Add a helper to allow the CLOSID allocator to check if a CLOSID has dirty
>> RMID values. This behaviour is enabled by a kconfig option selected by
>> the architecture, which avoids a pointless search for x86.

>> +/**
>> + * resctrl_closid_is_dirty - Determine if clean RMID can be allocate for this
>> + *                           CLOSID.
>> + * @closid: The CLOSID that is being queried.
>> + *
>> + * MPAM's equivalent of RMID are per-CLOSID, meaning a freshly allocate CLOSID
>> + * may not be able to allocate clean RMID. To avoid this the allocator will
>> + * only return clean CLOSID.
>> + */
>> +bool resctrl_closid_is_dirty(u32 closid)
>> +{
>> +    struct rmid_entry *entry;
>> +    int i;
>> +
>> +    lockdep_assert_held(&rdtgroup_mutex);
>> +
>> +    if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID))
>> +        return false;
> 
> Since dirty closid occurs when the kconfig option for MPAM is enabled, it seems
> that the condition of the if statement here should take the opposite value:
>     if (!IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID))

Yup. Bother.

Thanks for spotting that! It was intended to avoid this work on x86 as its pointless, and
the number of RMID could be large.


Thanks!

James