Re: [BUG] sched/cache: "Make LLC id continuous" causes NULL cpumask

["Chen, Yu C" <yu.c.chen@intel.com>]

Hi Shrikanth,

On 5/27/2026 3:01 PM, Shrikanth Hegde wrote:
> Hi Chen, Prateek.
> 
> I got back to work today, sorry for delay.
> I am trying to go through the mails.
> Apologies in case i have missed any bits.
> 

Thanks for taking a look at this!

> On 5/26/26 7:38 PM, Chen Yu wrote:
>> Hi Prateek,
>>
>> On Tue, 26 May 2026 11:23:59 +0530, K Prateek Nayak 
>> <kprateek.nayak@amd.com> wrote:
>>> Hello Srikar,
>>>
>>> On 5/26/2026 10:28 AM, Srikar Dronamraju wrote:
>>>> L2 Cache reported here is for SMT8 Core aka CACHE domain.
>>>
>>> Apart for the scheduler, nothing in tree currently cares about
>>> cpu_coregroup_mask() except for drivers/base/arch_topology.c but
>>> Power doesn't select GENERIC_ARCH_TOPOLOGY.
>>>
>>> Why can't Power have an internal mask for MC domain (tl_mc_mask) and
>>> the scheduler can use cpu_coregroup_mask() for the actual LLc? (The L2
>>> mask in this case.)
> 
> This seems wrong. there is no notion that coregroup_mask
> (MC domain) has to point at LLC domain.
> 
> For example, on Shared LPAR, there is no MC domain and LLC is at SMT 
> core level.
> In that case coregroup_mask has point at SMT mask is wrong.
>

On Shared LPAR, highest_flag_domain(SD_SHARE_LLC) selected the
SMT domain(L2 shared)prior to commit b5ea300a17e3.
Prateek suggested changing cpu_coregroup_mask() to use
cpu_l2_cache_mask(), which makes the LLC mask cover the same range.
sd_llc, size and grouping remain unchanged. Only sd_llc_id becomes
contiguous, which aligns with the intent of this commit.

But yes, the naming is confusing. cpu_coregroup_mask suggests a
"group of cores", but after the change, it only covers threads
within a single SMT core.

> If we need a mask to point to the LLC mask which arch has to return, 
> then we would
> need a new api say cpu_llc_mask ? that can point accordingly.
> 

Do you mean something like this?
https://lore.kernel.org/lkml/8d14c844-b4a8-4af6-acab-2cfdd42225be@intel.com/

> I don't like mixing MC domain and LLC into one bit.
> 

[ ... ]

>>   struct cpumask *cpu_coregroup_mask(int cpu)
>>   {
>> -    return per_cpu(cpu_coregroup_map, cpu);
>> +    return cpu_l2_cache_mask(cpu);
>> +}
> 
> This looks wrong to me too. In different hardware topologies
> there maybe distinction between coregroup and l2 mask.
> 
> Let me go through the code and see if there is better way.
> 

Sure, please go ahead - I'm on board with the direction
you settle on.

thanks,
Chenyu