Re: [PATCH v4 2/3] fs/resctrl: Assign counters to existing groups when enabling mbm_event

[Ben Horgan <ben.horgan@arm.com>]

Hi Babu,

On 10/10/2026 00:44, Babu Moger wrote:
> When the user switches counter assignment mode by writing "mbm_event" to
> /sys/fs/resctrl/info/L3_MON/mbm_assign_mode, resctrl frees all assignable
> counters, resets per-domain RMID state, and enables counter auto-assignment
> exposed to user space as mbm_assign_on_mkdir.
> 
> Even though counter auto-assignment is enabled, groups that already exist
> at the time of the switch, including the default group created at mount,
> are not assigned a counter. All MBM events read "Unassigned" until the user
> assigns a counter by hand.
> 
> Walk every existing CTRL_MON and its MON children after the reset and
> assign counters to their MBM events. Enabling "mbm_event" now leaves the
> same per-group state that auto-assignment would have produced. There may be
> fewer available counters than MBM events across the existing groups; in
> that case stop assignment when no counters remain. Events in the remaining
> groups read "Unassigned", matching the behavior of creating a group when no
> counters are available. Also revise the rdtgroup_assign_cntrs() function
> comments to account for the new callers.
> 
> Fixes: 8004ea01cf63 ("fs/resctrl: Introduce the interface to switch between monitor modes")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/patchset/8cb66e18e32e4087a9712c1e68ee6da614efe244.1784322818.git.babu.moger%40amd.com
> Signed-off-by: Babu Moger <babu.moger@amd.com>
> Cc: stable@vger.kernel.org
> ---
> v4: Changelog update (Reinette's input).
>     Function comment update for rdtgroup_assign_cntrs().
> 
> v3: Changelog update.
>     Code comment cleanup.
>     Added Cc to stable.
>     Added Reported-by and Closes tags.
> 
> v2: New patch.
>     This patch addresses the Sashiko comment about documentation issue where
>     counters are not assigned automatically when mode is switched to mbm_event.
>     https://sashiko.dev/#/patchset/8cb66e18e32e4087a9712c1e68ee6da614efe244.1784322818.git.babu.moger%40amd.com
>     In fact, it exposed a real issue. When switching to mbm_event mode, existing
>     monitoring groups should be assigned counters whenever counters are available.
>     This provides a smooth transition between modes and aligns the behavior with
>     the existing auto-assignment mechanism.


Given the name of the option is 'mbm_assign_on_mkdir' I expected the auto assignment to only occur
on the creation of ctrl groups or monitor groups. That's not my concern though. As you are now
treating this as more of a general auto assignment should the assignment also be done when a resctrl
domain is brought back online.

For illustration on an MPAM model I see:

/sys/fs/resctrl# cat mbm_L3_assignments
mbm_total_bytes:2=e;3=e

# offline one domain
/sys/fs/resctrl# for i in $(seq 0 3); do echo 0 > /sys/devices/system/cpu/cpu$i/online; done
/sys/fs/resctrl# cat mbm_L3_assignments
mbm_total_bytes:3=e

# bring the domain back online
/sys/fs/resctrl# cat mbm_L3_assignments
mbm_total_bytes:2=_;3=e # Would it be better, or worse, for auto-assignment to occur here?

Thanks,

Ben