linux-kernel - Re: [PATCH v10 07/30] x86/resctrl: Check all domains are offline in resctrl

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Message-ID: <abdd65a7-1f8e-4c3f-8e08-94787a054430@intel.com>
Date: Fri, 9 May 2025 09:28:59 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Catalin Marinas <catalin.marinas@....com>
CC: James Morse <james.morse@....com>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>, 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>, <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>,
	David Hildenbrand <david@...hat.com>, Rex Nie <rex.nie@...uarmicro.com>,
	"Dave Martin" <dave.martin@....com>, Koba Ko <kobak@...dia.com>, Shanker
 Donthineni <sdonthineni@...dia.com>, <fenghuay@...dia.com>, Shaopeng Tan
	<tan.shaopeng@...fujitsu.com>
Subject: Re: [PATCH v10 07/30] x86/resctrl: Check all domains are offline in
 resctrl_exit()

Hi Catalin,

On 5/9/25 9:17 AM, Reinette Chatre wrote:
> Hi Catalin,
> 
> On 5/9/25 4:21 AM, Catalin Marinas wrote:
>> Hi Reinette,
>>
>> Thanks for the reviews.
>>
>> On Thu, May 08, 2025 at 10:51:23AM -0700, Reinette Chatre wrote:
>>> On 5/8/25 10:18 AM, James Morse wrote:
>>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>> index 88197afbbb8a..f617ac97758b 100644
>>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>> @@ -4420,8 +4420,32 @@ int __init resctrl_init(void)
>>>>  	return ret;
>>>>  }
>>>>  
>>>> +static bool __exit resctrl_online_domains_exist(void)
>>>> +{
>>>> +	struct rdt_resource *r;
>>>> +
>>>> +	for_each_rdt_resource(r) {
>>>> +		if (!list_empty(&r->ctrl_domains) || !list_empty(&r->mon_domains))
>>>> +			return true;
>>>> +	}
>>>> +
>>>> +	return false;
>>>> +}
>>>
>>> This looks the same as before. Did you notice my comment in [1] about this list_empty() usage?
>>
>> James may have missed that comment (and he's off today). Copying your
>> comment here:
>>
>>> A list needs to be initialized for list_empty() to behave as intended. A list within
>>> an uninitialized or "kzalloc()'ed" struct will not be considered empty.
>>> resctrl_arch_get_resource() as used by for_each_rdt_resource() already establishes
>>> that if an architecture does not support a particular resource then it can (should?)
>>> return a "dummy/not-capable" resource. I do not think resctrl should require
>>> anything additionally like initializing the lists of a dummy/not-capable resource
>>> to support things like this loop.
>>
>> I agree the description of the resctrl_arch_get_resource() semantics
>> doesn't specifically mention that the list_heads should be initialised
>> in dummy resources. IIUC, x86 should be safe for now since the
>> rdt_resources_all[] array elements have the ctrl_domains and mon_domains
>> list_heads initialised.
> 
> Not all x86 resources support both control and monitoring. For these resources x86
> only initializes the arrays it uses. For example, the L2 resource only supports control
> and thus only the ctrl_domains list is initialized. When the above loop runs on a resource
> like this it will return "true" because it believes there are monitoring domains
> online.
> 
> Your conclusion that x86 is safe for now may (reason for use of "may" follows) still
> be valid since resctrl_online_domains_exist() is only called by x86 from its
> resctrl_arch_exit() that is within the "exit.text" section. Here is where I am not
> entirely certain of the impact (hence the earlier use of "may") since from what I can
> tell the linker does not actually discard "exit.text" on x86 because it defines
> RUNTIME_DISCARD_EXIT.

If you are not familiar with the details then I should add for completeness that resctrl
is not currently capable of being compiled as a module.

Reinette