Re: [RFC PATCH 0/8] arm,x86,fs/resctrl: Support asymmetric L2 cache allocation

[Luck, Tony]

On Mon, Oct 26, 2026 at 03:30:57PM -0700, Reinette Chatre wrote:
> Hi Tony,
> 
> On 10/26/26 2:57 PM, Tony Luck wrote:
> > RFC patch series built on top of Reinette's multiple control series[1]
> > 
> > The goal is to support L2 cache allocation on hybrid Intel CPUs such as
> > Panther Lake[2] where the P-core and E-core CPUs have different number
> > of supported bits for L2 cache bit mask controls. N.B. it is assumned
> > that the only point of asymmetry is the cache bit mask length. Other
> > parameters must be the same (but code doesn't check).
> > 
> > Overall approach is to add the per-L2 cache bitmask length to the domain
> > and use that everywhere instead of using a single value from the control.
> > 
> > Patch 1 changes the format of the per-control "max" file and should be
> > merged back into Reinette's multi-control series so this file can have
> > a consistent format from first upstream introduction.
> > 
> > Patches 2-5 are enumeration and preperation.
> > 
> > Patch 6 introduces, initializes, and uses the per-domain cache bit mask
> > length field (arm/MPAM change needs careful scrutiny).
> > 
> > Patch 7 enables asymmetric L2 for x86 CPUs that enumerate it. N.B. it
> > renammes the resource from "L2" to "L2A" to avoid user interface issues
> > with tools that expect L2 cache allocation to be symmetric.
> 
> I did a quick scan and it seems to ignore the the distinction between a resource
> and control as highlighted the previous time you brought up this work. Specifically,
> please see:
> 
> 	https://lore.kernel.org/lkml/eaeb4259-46d6-4656-8354-669ad32654b5@xxxxxxxxx/

I think I see now. You want the "asymmetry" to be a property of the
control, not of the resource.

So the resource name stays as "L2" but the control gets a suffix (e.g.  "_ASYM")

File system looks like this:

# tree
.
├── cpus
├── cpus_list
├── info
│   ├── L2
│   │   ├── bit_usage
│   │   ├── control_mode
│   │   ├── min_cbm_bits
│   │   ├── num_closids
│   │   ├── schemata
│   │   │   └── L2_ASYM
│   │   │       ├── max
│   │   │       ├── min_bits
│   │   │       └── type
│   │   ├── shareable_bits
│   │   └── sparse_masks
│   └── last_cmd_status
├── mode
├── schemata
├── size
└── tasks

and the schemata file:

# cat schemata
    L2_ASYM:0=fff;1=fff;2=fff;3=fff;4=ffff;5=ffff;8=ffff

> 
> Also, why does patch #1 drop the 0x prefix from new control interface when displaying hex
> values to user space? This is a significant change for which patch #1 does not provide
> a motivation.

Sorry for missing out the justification. I thought that matching the
format of the schemata file would be more useful. Then someone could
reset a group to maximum values with:

# echo L2_ASYM:$(cat /sys/fs/resctrl/info/L2/schemata/L2_ASYM/max) >  schemata

But if you like the "0x" prefixes I'm OK with that too. Just need to
replace "cat" with "sed -e s/0x//g" to reset to maximum values.
> 
> Reinette

-Tony