linux-kernel - Re: [RFC PATCH 07/38] arm_mpam: resctrl: Add boilerplate cpuhp and domain allocation

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Message-ID: <7ca41a6a-60c1-486f-a0f8-4af256fa1b4a@arm.com>
Date: Fri, 2 Jan 2026 11:07:05 +0000
From: Ben Horgan <ben.horgan@....com>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: James Morse <james.morse@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org,
 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@...nel.org>, Dave Martin <dave.martin@....com>,
 Koba Ko <kobak@...dia.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 fenghuay@...dia.com, baisheng.gao@...soc.com, Gavin Shan <gshan@...hat.com>,
 rohit.mathew@....com, reinette.chatre@...el.com,
 Punit Agrawal <punit.agrawal@....qualcomm.com>
Subject: Re: [RFC PATCH 07/38] arm_mpam: resctrl: Add boilerplate cpuhp and
 domain allocation

Hi Jonathan,

On 12/22/25 11:48, Jonathan Cameron wrote:
> 
>>>> +static struct mpam_resctrl_dom *
>>>> +mpam_resctrl_get_domain_from_cpu(int cpu, struct mpam_resctrl_res *res)
>>>> +{
>>>> +	struct mpam_resctrl_dom *dom;
>>>> +	struct rdt_ctrl_domain *ctrl_d;
>>>> +
>>>> +	lockdep_assert_cpus_held();
>>>> +
>>>> +	list_for_each_entry_rcu(ctrl_d, &res->resctrl_res.ctrl_domains,
>>>> +				hdr.list) {
>>>> +		dom = container_of(ctrl_d, struct mpam_resctrl_dom,
>>>> +				   resctrl_ctrl_dom);  
>>>
>>> I'm lazy so haven't checked for more code here in later patches, but
>>> if not, why not iterate the list to access the domain directly rather
>>> than jumping through the rdt_ctrl_domain?
>>>
>>> Something along lines of:
>>> 	
>>> 	list_for_each_entry_rcu(dom, &res->resctrl_res.ctrl_domains,
>>> 				resctrl_ctrl_dom.hdr.list) {
>>> 	}
>>>  
>>
>> Unless I've misunderstood I don't think this works because it's not what
>> the fs/resctrl code expects.
> 
> I think I explained this one badly.
> 
> This should be functionally identical to the above so no visible side
> effects outside of this code.  All this change is meant to do is wrap the
> container_of() in the list iterator. When using the _entry_ variants
> it is wrapping container_of() anyway so just going one level further
> up the hierarchy of nested structures.
> 
> struct a {
> 	struct b {
> 		struct list_head l;
> 	}
> }
> 
> It's actually a list of struct a as all elements on this list are
> struct b instances within struct a, but you are treating it as a list
> of struct b and then using a container_of() to get to struct a on
> each one.
> 
> The change is treat it as a list of struct a with the list_head happening
> to be wrapped in struct b.  Results in slightly simpler code and makes
> the point these are always struct a instances.

Thanks for the detailed explanation. This makes sense to me now and I'll
make the change.

> 
> Jonathan
> 
> 
> 
>>
>>
>>>> +
>>>> +		if (cpumask_test_cpu(cpu, &dom->ctrl_comp->affinity))
>>>> +			return dom;
>>>> +	}
>>>> +
>>>> +	return NULL;
>>>> +}
>>>> +
> 
> 

Thanks,

Ben