linux-kernel - Re: [PATCH v3 12/19] x86/resctrl: Make resctrl

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Message-ID: <50775fa5-2da5-90fa-5f6c-9a12b8364e9b@arm.com>
Date:   Thu, 25 May 2023 18:31:56 +0100
From:   James Morse <james.morse@....com>
To:     Reinette Chatre <reinette.chatre@...el.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 12/19] x86/resctrl: Make resctrl_mounted checks
 explicit

Hi Reinette,

On 28/04/2023 00:37, Reinette Chatre wrote:
> On 4/27/2023 7:19 AM, James Morse wrote:
>> On 01/04/2023 00:28, Reinette Chatre wrote:
>>> On 3/20/2023 10:26 AM, James Morse wrote:
>>>> The rdt_enable_key is switched when resctrl is mounted, and used to
>>>> prevent a second mount of the filesystem. It also enables the
>>>> architecture's context switch code.
>>>>
>>>> This requires another architecture to have the same set of static-keys,
>>>> as resctrl depends on them too.
>>>>
>>>> Make the resctrl_mounted checks explicit: resctrl can keep track of
>>>> whether it has been mounted once. This doesn't need to be combined with
>>>> whether the arch code is context switching the CLOSID.
>>>> Tests against the rdt_mon_enable_key become a test that resctrl is
>>>> mounted and that monitoring is enabled.
>>>
>>> The last sentence above makes the code change hard to follow ...
>>> (see below)
>>>
>>>> This will allow the static-key changing to be moved behind resctrl_arch_
>>>> calls.
>>
>>>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>>>> index f38cd2f12285..6279f5c98b39 100644
>>>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>>>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
>>>> @@ -834,7 +834,7 @@ void mbm_handle_overflow(struct work_struct *work)
>>>>  
>>>>  	mutex_lock(&rdtgroup_mutex);
>>>>  
>>>> -	if (!static_branch_likely(&rdt_mon_enable_key))
>>>> +	if (!resctrl_mounted || !static_branch_likely(&rdt_mon_enable_key))
>>>
>>> ... considering the text in the changelog the "resctrl_mounted" check seems
>>> unnecessary. Looking ahead I wonder if this check would not be more
>>> appropriate in patch 15?
>>
>> How so?
>>
>> This is secretly relying on rdt_mon_enable_key being cleared in rdt_kill_sb() when the
>> filesystem is unmounted, otherwise the overflow thread keeps running once the filesystem
>> is unmounted.
> 
> hmmm ... I do not think my feedback was clear. I understand that this is done
> as a prep patch but that was only clear when I read patch 15 because as the
> work is presented here it seems unnecessary. 
> 
>>
>> I thought it simpler to add all these checks explicitly in one go.
>> That makes it simpler to thin out the static keys as their 'and its mounted' behaviour is
>> no longer relied on.
> 
> Understood. If you want to keep this as a prep patch, could you please update the
> changelog to reflect this? The following sentence in the changelog makes this patch
> hard to follow since it essentially claims that what this patch does is unnecessary:
> "Tests against the rdt_mon_enable_key become a test that resctrl is mounted
> and that monitoring is enabled."

"Because of the implicit mount test" ... the text immediately before this.

We're probably going to keep talking past each other on this - I'll rephrase that
paragraph as:
| rdt_mon_enable_key is never used just to test that resctrl is mounted,
| but does also have this implication. Add a resctrl_mounted to all uses
| of rdt_mon_enable_key. This will allow rdt_mon_enable_key to be swapped
| with a helper in a subsequent patch.


> I also do still wonder why these resctrl_mounted checks cannot move to patch
> 15 when they are needed. Adding them there makes it obvious that rdt_mon_enable_key
> had a dual purpose that patch 15 splits into two separate checks. 

That is happening in this patch too, rdt_mon_enable_key becomes
(resctrl_mounted && rdt_mon_enable_key), the implicit property is now explicit, so a later
patch can modify rdt_mon_enable_key without breaking this behaviour.

I think its easier to review if patch 15 is making a set of 1:1 mappings instead of
splitting some static-keys but not others. Let me know what you think of the new version.


Thanks,

James