Re: [PATCH v7 2/5] riscv_cbqri: resctrl: Add cache allocation via capacity block mask

[Reinette Chatre]

Hi Drew,

On 9/1/26 3:31 PM, Drew Fustini wrote:
> resctrl exposes a single cap set per rid, so when more than one capacity
> controller backs a cache level they must agree on those caps. A level
> whose controllers disagree is left unexposed. Other levels are
> unaffected.
If I understand correctly, while attempting to translate to resctrl terms,
this means that a cache resource with several domains may support different
capacity masks in the domains?

I believe the "single cap set per rid" you refer to is
/sys/fs/resctrl/info/L3/cbm_mask and /sys/fs/resctrl/info/L2/cbm_mask that
exposes a single capacity bitmask representing the maximum allocation.

As you know, resctrl is in the process of changing the way in which schema
formats are described. The most recent PoC [1] added new bitmap description
files but it kept a single value "max" file to describe the highest allocation
possible for a bitmap schema.

To support asymmetrical cache instances it looks like changing the interface
to describe the maximum per domain (instead of one global value) could address
the gap?

For example, if there is a hypothetical system with two L2 cache instances, one
with a 0xFF max CBM and the other with a 0XF max CBM then resctrl could expose it
as:

	# cat /sys/fs/resctrl/info/L2/schemata/L2/max
	0=ff;1=f

The schemata file currently and will continue to display the allocations per domain.

resctrl will need more changes to support different CBM per domain internally and
until then all these CBMs will be the same, but adding this change to user interface now
may set resctrl up to support these systems in the future?

Reinette

[1] https://lore.kernel.org/lkml/d258a32f-12d5-464d-abe9-4720fb3e44b3@xxxxxxxxx/