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

[Jinjie Ruan]

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@xxxxxxxxx>
> ---
>  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@xxxxxxxxxx>

> -
> +	ctrl_comp = find_component(class, cpu);
>  	/* class has no component for this CPU */
>  	if (WARN_ON_ONCE(!ctrl_comp))
>  		return ERR_PTR(-EINVAL);

-- 
Best regards,
Jinjie