Re: [PATCH v3 04/21] sched/cache: Make LLC id continuous

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

On 2/17/2026 4:09 PM, K Prateek Nayak wrote:
> Hello Chenyu,
> 
> On 2/17/2026 11:37 AM, Chen, Yu C wrote:
>> Hi Prateek,
>>
>> On 2/16/2026 3:44 PM, K Prateek Nayak wrote:
>>> Hello Tim, Chenyu,
>>>
>>> On 2/11/2026 3:48 AM, Tim Chen wrote:
>>>> From: Chen Yu <yu.c.chen@intel.com>

[...]

>>>> @@ -2567,10 +2567,18 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>>>>          /* Set up domains for CPUs specified by the cpu_map: */
>>>>        for_each_cpu(i, cpu_map) {
>>>> -        struct sched_domain_topology_level *tl;
>>>> +        struct sched_domain_topology_level *tl, *tl_llc = NULL;
>>>> +        int lid;
>>>>              sd = NULL;
>>>>            for_each_sd_topology(tl) {
>>>> +            int flags = 0;
>>>> +
>>>> +            if (tl->sd_flags)
>>>> +                flags = (*tl->sd_flags)();
>>>> +
>>>> +            if (flags & SD_SHARE_LLC)
>>>> +                tl_llc = tl;
>>>
>>> nit. This loop breaks out when sched_domain_span(sd) covers the entire
>>> cpu_map and it might have not reached the topmost SD_SHARE_LLC domain
>>> yet. Is that cause for any concern?
>>>
>>
>> Could you please elaborate a little more on this? If it covers the
>> entire cpu_map shouldn't it stop going up to its parent domain?
>> Do you mean, sd_llc_1 and its parent sd_llc_2 could cover the same cpu_map,
>> and we should let tl_llc to assigned to sd_llc_2 (sd_llc_1 be degenerated? )
> 
> I'm not sure if this is technically possible but assume following
> topology:
> 
>     [                 LLC: 8-15                 ]
>     [ SMT: 8,9 ][ SMT: 10,11 ] ... [ SMT: 14,15 ]
> 
> and the following series of events:
> 
> o All CPUs in LLC are offline to begin with (maxcpus = 1 like scenario).
> 
> o CPUs 10-15 are onlined first.
> 
> o CPU8 is put in a separate root partition and brought online.
>    (XXX: I'm not 100% sure if this is possible in this order)
> 

It can happen, I had a try on VM, and there would be different
llc_id within 1 LLC even after CPU8 has been taken out of the separate
partition.

> o build_sched_domains() will bail out at SMT domain since the cpumap
>    is covered by tl->mask() and tl_llc = tl_smt.
> 
> o llc_id calculation uses the tl_smt->mask() which will not contain
>    CPUs 10-15 and CPU8 will get a unique LLC id even though there are
>    other online CPUs in the LLC with a different llc_id (!!!)
> 
> 

Fair enough, thanks for the explanation in detail.

thanks,
Chenyu