Re: [PATCH v10 03/17] fs/resctrl: Remove redundant calls to resctrl_arch_mon_capable()

[Reinette Chatre]

Hi Tony,

On 7/29/26 10:27 AM, Tony Luck wrote:
> Architecture code provides resctrl_arch_mon_capable() so that file
> system code knows whether any monitor features are supported. This
> is used to decide whether to create "mon_data" directories etc.
> 
> Initially resctrl_arch_mon_capable() was an inline function providing
> the value of the x86 architecture variable "rdt_mon_capable". I.e.
> extremely low overhead and so it was used liberally as a sanity check.
> 
> The ARM implementation of resctrl_arch_mon_capable() is not inline and
> chases a couple of pointers. Future changes to x86 implementation will
> also make resctrl_arch_mon_capable() more expensive.
> 
> Redundant calls to resctrl_arch_mon_capable() in hot code paths should
> be eliminated.
> 
> 1) mbm_handle_overflow()
> This function is called once per second from worker threads running on
> each L3 domain. The call is clearly redundant because worker threads are
> only created if the MBM monitoring feature is enabled.
> 
> 2) is_rmid_match()
> This function is called when a user reads a "tasks" file of a MON_GROUP.

Above sentence can just be dropped since it is incomplete wrt where is_rmid_match()
is used. If you want to add a motivation it may be supportive to mention
that all callers of is_rmid_match() call it for every task on the system so
making it less expensive is appealing.

> The call is redundant because the function also checks "r->type ==
> RDTMON_GROUP" and this type of group can only be created on systems that
> support monitoring.
> 
> Remove the call to resctrl_arch_mon_capable() from both of these
> functions.
> 
> Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
> ---

| Reviewed-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>

Reinette