linux-kernel - Re: [PATCH v6 11/24] x86/resctrl: Move CLOSID/RMID matching and setting to use helpers

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Message-ID: <94a80f47-d28d-5638-1492-f1b4a3e3410c@intel.com>
Date:   Tue, 3 Oct 2023 14:15:10 -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>,
        <dfustini@...libre.com>, <amitsinght@...vell.com>
Subject: Re: [PATCH v6 11/24] x86/resctrl: Move CLOSID/RMID matching and
 setting to use helpers

Hi James,

On 9/14/2023 10:21 AM, 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.
> 
> Because MPAM's RMID values are not unique, the CLOSID must be provided
> when matching the RMID.
> 
> Reviewed-by: Shaopeng Tan <tan.shaopeng@...itsu.com>
> Tested-by: Shaopeng Tan <tan.shaopeng@...itsu.com>
> Tested-By: Peter Newman <peternewman@...gle.com>
> Signed-off-by: James Morse <james.morse@....com>
> ---

Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Reinette