Re: [RFC PATCH v2 01/10] arm_mpam: resctrl: Use find_component() when allocating a domain

[Reinette Chatre <reinette.chatre@intel.com>]

On 10/23/26 1:07 AM, Jinjie Ruan wrote:
> 
> 
> 在 2026/10/20 6:07, Reinette Chatre 写道:
>> mpam_resctrl_alloc_domain() starts by checking that the resource's
>> class has a component whose CPU affinity includes the CPU, and fails the
>> domain allocation otherwise. When the resource supports allocation, that
>> component also backs the control domain. The check uses an open-coded
>> walk of the class's component list, while the same function uses
>> find_component() to find the component backing each monitoring event.
>>
>> The open-coded walk duplicates find_component(). It also holds the SRCU
>> read lock until mpam_resctrl_alloc_domain() returns, which suggests that
>> the component needs this protection while the domain is set up.
>>
>> The component does not need it. Components are freed only when an MSC
>> fails to probe, in which case MPAM is never enabled and no resctrl
>> domain is created, or by mpam_disable(), which first removes the CPU
>> hotplug callbacks and thereby tears down all resctrl domains. A component
>> therefore outlives every domain that refers to it. The SRCU read lock is
>> only needed while walking the component list, and find_component() holds
>> it for that walk. In addition, mpam_resctrl_alloc_domain() only runs
>> from MPAM's CPU hotplug startup callback, which already holds the SRCU
>> read lock throughout.
>>
>> Replace the open-coded walk with find_component().
>>
>> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
>> ---
>>  drivers/resctrl/mpam_resctrl.c | 13 ++-----------
>>  1 file changed, 2 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
>> index 9cec5843e8fa..3d5f90f9f1f3 100644
>> --- a/drivers/resctrl/mpam_resctrl.c
>> +++ b/drivers/resctrl/mpam_resctrl.c
>> @@ -1623,21 +1623,12 @@ mpam_resctrl_alloc_domain(unsigned int cpu, struct mpam_resctrl_res *res)
>>  	struct rdt_l3_mon_domain *mon_d;
>>  	struct rdt_ctrl_domain *ctrl_d;
>>  	struct mpam_class *class = res->class;
>> -	struct mpam_component *comp_iter, *ctrl_comp;
>> +	struct mpam_component *ctrl_comp;
>>  	struct rdt_resource *r = &res->resctrl_res;
>>  
>>  	lockdep_assert_held(&domain_list_lock);
>>  
>> -	ctrl_comp = NULL;
>> -	guard(srcu)(&mpam_srcu);
>> -	list_for_each_entry_srcu(comp_iter, &class->components, class_list,
>> -				 srcu_read_lock_held(&mpam_srcu)) {
>> -		if (cpumask_test_cpu(cpu, &comp_iter->affinity)) {
>> -			ctrl_comp = comp_iter;
>> -			break;
>> -		}
>> -	}
> 
> LGTM
> Reviewed-by: Jinjie Ruan <ruanjinjie@huawei.com>
> 
Thank you very much for taking a look at this series.

Reinette