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

[Reinette Chatre]

Hi Drew,

On 9/17/26 9:18 AM, Drew Fustini wrote:
> On Thu, Sep 17, 2026 at 08:37:43AM -0700, Reinette Chatre wrote:
>> 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?
> 
> Yes, each capacity controller reports its own number of capacity blocks,
> so two controllers backing the same cache level can have different
> maximum capacity masks.

Thank you.

> 
>> 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?
> 
> I think that would be a good change to the generic schema rfc as a max
> per domain would be helpful for CBQRI. The hardware I am working with

Will do.

> right now (Tenstorrent Ascalon) does not have two controllers at one
> level, but I think it is reasonable to assume other RISC-V platforms may
> do that in the future.

Looking at this patch more it seems the controllers could also differ in
CDP ("supports_alloc_at_code") support. Supporting that in resctrl is a
different story. 
> BTW, I have been meaning to send v8 with some unrelated minor
> improvements based on sashiko comments in v7. I will go ahead and send
> right now in case you are looking at the rest of the series.
That is fine. I just started looking at v7 and will switch to v8 instead.

Thank you.

Reinette