linux-kernel - Re: [PATCH v7 15/24] x86/resctrl: Allow arch to allocate memory needed in resctrl_arch_rmid

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Message-ID: <08664556-76c3-51d9-4202-132298cf2d7b@arm.com>
Date:   Thu, 14 Dec 2023 11:38:02 +0000
From:   James Morse <james.morse@....com>
To:     babu.moger@....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>,
        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,
        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 v7 15/24] x86/resctrl: Allow arch to allocate memory
 needed in resctrl_arch_rmid_read()

Hi Babu,

On 09/11/2023 20:47, Moger, Babu wrote:
> On 10/25/23 13:03, James Morse wrote:
>> Depending on the number of monitors available, Arm's MPAM may need to
>> allocate a monitor prior to reading the counter value. Allocating a
>> contended resource may involve sleeping.
>>
>> add_rmid_to_limbo() calls resctrl_arch_rmid_read() for multiple domains,
>> the allocation should be valid for all domains.
> 
> This above sentence does not look correct after your previous patch [patch
> 14].  You removed  resctrl_arch_rmid_read() from add_rmid_to_limbo().

Ah, this was the justification for making the allocated resource global, when the patches
were in a different order. It's also simpler to make them global. I've removed this
paragraph, and replaced it with:
| The memory or hardware allocated is not specific to a domain.

(its fairly obvious from the interface, but I felt it needed calling out in the commit
message)

> Otherwise looks good.
> 
>>
>> __check_limbo() and mon_event_count() each make multiple calls to
>> resctrl_arch_rmid_read(), to avoid extra work on contended systems,
>> the allocation should be valid for multiple invocations of
>> resctrl_arch_rmid_read().
>>
>> Add arch hooks for this allocation, which need calling before
>> resctrl_arch_rmid_read(). The allocated monitor is passed to
>> resctrl_arch_rmid_read(), then freed again afterwards. The helper
>> can be called on any CPU, and can sleep.

> Reviewed-by: Babu Moger <babu.moger@....com>

Thanks!

James