Re: [RESEND PATCH v4 04/15] fs/resctrl: Introduce kernel mode (kmode) data structures

[Reinette Chatre]

Hi Babu,

On 7/7/26 2:50 PM, Babu Moger wrote:
> Kernel-mode traffic can use a different allocation and monitoring context
> than the originating user task. On x86, Privilege Level Zero Association
> (PLZA) enables the kernel to switch to a different CLOSID (and optionally
> RMID) when entering kernel mode.
> 
> Architectures need a common way to name kernel-mode policies before resctrl

Please pick one term and stick with it. While reading through this series I
have come across "kernel-mode policies", "kernel-mode binding", "kernel-mode
configuration", and "kernel-mode association". Is the distinction even needed?
Could all of these just instead be: "kernel mode"?

> can report what is active or what the platform supports.
> 
> Introduce enum resctrl_kernel_mode:
>   - INHERIT_CTRL_AND_MON: Kernel work inherits allocation and monitoring
>     from the user task (current behavior).

The issue with this "combination" mode becomes obvious in patch 7. I also
see that sashiko hinted at this issue but I am not able to see from your response
what the plan is to address this. As highlighted by patch 7 and sashiko the
"allocation" and "monitoring" features of a system are independent - a system
need not support/enable both. This should be easy to reproduce by, for example,
booting a system with needed rdt= options disabling allocation or monitoring
features.

I think it will be unexpected to a user on an allocation-only system to
see interface like:

 # cat info/kernel_mode
  [inherit_ctrl_and_mon]
  global_assign_ctrl_inherit_mon_per_cpu:group=uninitialized
  global_assign_ctrl_assign_mon_per_cpu:group=uninitialized

Should it not rather be, for example:
 # cat info/kernel_mode
  [inherit_ctrl]
  global_assign_ctrl_per_cpu:group=uninitialized

Similarly the user input would not need to provide a monitor group when system
only supports allocation.

>   - GLOBAL_ASSIGN_CTRL_INHERIT_MON_PER_CPU: Assign allocation for kernel
>     work; inherit monitoring from the user task.

I am not able to parse "Assign allocation for kernel work" 


>   - GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU: Assign a dedicated allocation
>     and monitoring for kernel work.
> 
> Signed-off-by: Babu Moger <babu.moger@xxxxxxx>
> ---

...

> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index 73ff522448a0..c7abed51cd5f 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -703,6 +703,37 @@ int resctrl_arch_io_alloc_enable(struct rdt_resource *r, bool enable);
>   */
>  bool resctrl_arch_get_io_alloc_enabled(struct rdt_resource *r);
>  
> +/**
> + * enum resctrl_kernel_mode - Kernel-mode control and monitor association
> + *			      policy.
> + *
> + * @INHERIT_CTRL_AND_MON:
> + *	Kernel work inherits the allocation and monitoring from the user space

"inherits the allocation and monitoring" is very vague. I think it will help to
make things clear if this is described as the control and monitor groups being
assigned.

> + *	task. On x86 this means that kernel work shares the same CLOSID and
> + *	RMID as the user space task. This matches today's resctrl behavior.

"This matches today's resctrl behavior." - This cannot be expected to age well and can
be dropped.

> + *
> + * @GLOBAL_ASSIGN_CTRL_INHERIT_MON_PER_CPU:
> + *	Kernel work uses a globally assigned allocation while monitoring is
> + *	inherited from the user space task. On x86 this means a CLOSID is

(same comment as above)

> + *	assigned for kernel work and the RMID is inherited from the user space
> + *	task. Default scope is all online CPUs; a subset may be selected via
> + *	the resctrl group interface. A CTRL_MON group is bound to this mode.
> + *
> + * @GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU:
> + *	Kernel work uses globally assigned allocation and monitoring. On x86

(same comment as above)

> + *	this means both CLOSID and RMID are assigned for kernel work. Default
> + *	scope is all online CPUs; a subset may be selected via the resctrl
> + *	group interface. A CTRL_MON or MON group is bound to this mode.
> + */
> +enum resctrl_kernel_mode {
> +	INHERIT_CTRL_AND_MON,
> +	GLOBAL_ASSIGN_CTRL_INHERIT_MON_PER_CPU,
> +	GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU,
> +	RESCTRL_KMODE_LAST = GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU,

Please drop comma on a terminator line.

> +};
> +
> +#define RESCTRL_NUM_KERNEL_MODES (RESCTRL_KMODE_LAST + 1)
> +
>  extern unsigned int resctrl_rmid_realloc_threshold;
>  extern unsigned int resctrl_rmid_realloc_limit;
>  

Reinette