linux-kernel - Re: [PATCH 07/18] x86/resctrl: Move CLOSID/RMID matching and setting to use helpers

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Message-ID: <7a942c00-67a9-c038-b516-4ae85e997a6e@arm.com>
Date:   Thu, 24 Nov 2022 14:21:25 +0000
From:   James Morse <james.morse@....com>
To:     Valentin Schneider <vschneid@...hat.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 07/18] x86/resctrl: Move CLOSID/RMID matching and setting
 to use helpers

Hi Valentin,

On 18/11/2022 15:49, Valentin Schneider wrote:
> On 21/10/22 13:11, James Morse wrote:
>> When switching tasks, the CLOSID and RMID that the new task should
>> use are stored in struct task_struct. For x86 the CLOSID known by resctrl,
>> the value in task_struct, and the value written to the CPU register are
>> all the same thing.
>>
>> MPAM's CPU interface has two different PARTID's one for data accesses
>> the other for instruction fetch. Storing resctrl's CLOSID value in
>> struct task_struct implies the arch code knows whether resctrl is using
>> CDP.
>>
>> Move the matching and setting of the struct task_struct properties
>> to use helpers. This allows arm64 to store the hardware format of
>> the register, instead of having to convert it each time.
>>
>> __rdtgroup_move_task()s use of READ_ONCE()/WRITE_ONCE() ensures torn
>> values aren't seen as another CPU may schedule the task being moved
>> while the value is being changed. MPAM has an additional corner-case
>> here as the PMG bits extend the PARTID space. If the scheduler sees a
>> new-CLOSID but old-RMID, the task will dirty an RMID that the limbo code
>> is not watching causing an inaccurate count. x86's RMID are independent
>> values, so the limbo code will still be watching the old-RMID in this
>> circumstance.
>> To avoid this, arm64 needs both the CLOSID/RMID WRITE_ONCE()d together.
>> Both values must be provided together.

> I think I remember something about this... Is that about having them
> union'd and read/written as one? (just for my own curiosity)

Yup. Many moons ago you pointed out that storing partid/pmg separately lets the remote
CPUs load torn values. arm64 not only needs the values as a pair, it needs to read/write
them as a pair.


>> Because MPAM's RMID values are not unique, the CLOSID must be provided
>> when matching the RMID.
>>
>> CC: Valentin Schneider <vschneid@...hat.com>

(CC here also stands for celebrate-contribution!)


>> Signed-off-by: James Morse <james.morse@....com>
> 
> Reviewed-by: Valentin Schneider <vschneid@...hat.com>

Thanks!

James