Re: [PATCH v10 07/30] x86/resctrl: Check all domains are offline in resctrl_exit()

[James Morse <james.morse@arm.com>]

Hi Reinette,

On 09/05/2025 17:28, Reinette Chatre wrote:
> On 5/9/25 9:17 AM, Reinette Chatre wrote:
>> On 5/9/25 4:21 AM, Catalin Marinas wrote:
>>> 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.

I had missed this. The logic makes sense - I'll spin this as a v11.


>> 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.

Aha - I'd not spotted that one before.


> 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.


Thanks,

James