Re: [RFC PATCH 3/7] resctrl: Add a devices file for external requester assignment

[Drew Fustini <fustini@kernel.org>]

On Tue, Jul 14, 2026 at 09:06:53PM +0800, Zhanpeng Zhang wrote:
> Do not overload the resctrl tasks file with architecture-specific
> non-PID tokens. The tasks ABI remains a list of task IDs, while the new
> devices file carries external requesters assigned to a resctrl group.
> 
> Add architecture hooks for assigning and showing those external
> objects, and reject group removal, reparenting, or pseudo-lock setup
> while devices are still assigned. The teardown path performs a
> best-effort reset to the default group.
> 
> The devices file is an assignment interface only. It does not describe
> a new resource schema or domain; resource allocation and monitoring
> policy remain described by the existing schemata and info files.
> 
> Signed-off-by: Zhanpeng Zhang <zhangzhanpeng.jasper@bytedance.com>
> ---
>  Documentation/filesystems/resctrl.rst |  26 ++++
>  arch/Kconfig                          |   6 +
>  fs/resctrl/rdtgroup.c                 | 206 +++++++++++++++++++++++++-
>  include/linux/resctrl.h               |  45 ++++++
>  4 files changed, 280 insertions(+), 3 deletions(-)
[..]
> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
> index af2cbab14497..2e424c911049 100644
> --- a/fs/resctrl/rdtgroup.c
> +++ b/fs/resctrl/rdtgroup.c
[..]
> @@ -2067,6 +2236,16 @@ static struct rftype res_common_files[] = {
>  		.seq_show	= rdtgroup_tasks_show,
>  		.fflags		= RFTYPE_BASE,
>  	},
> +#ifdef CONFIG_ARCH_HAS_RESCTRL_DEVICES
> +	{
> +		.name		= "devices",
> +		.mode		= 0644,
> +		.kf_ops		= &rdtgroup_kf_single_ops,
> +		.write		= rdtgroup_devices_write,
> +		.seq_show	= rdtgroup_devices_show,
> +		.fflags		= RFTYPE_BASE,

Once CONFIG_ARCH_HAS_RESCTRL_DEVICES is built in, the file shows up in
every resctrl group directory, even on hardware whose IOMMU does not
implement QOSID. I think it would be better to make fflags zero so
rdtgroup_add_files() skips it, and enable it at runtime from
resctrl_init().

Thanks,
Drew