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

[K Prateek Nayak]

Hello Tim,

On 2/18/2026 4:42 AM, Tim Chen wrote:
> On Tue, 2026-02-17 at 13:39 +0530, K Prateek Nayak wrote:
>> Hello Chenyu,
>>
>>
> 
> [...snip...]
> 
> 
>>>>>    */
>>>>>   DEFINE_PER_CPU(struct sched_domain __rcu *, sd_llc);
>>>>>   DEFINE_PER_CPU(int, sd_llc_size);
>>>>> -DEFINE_PER_CPU(int, sd_llc_id);
>>>>> +DEFINE_PER_CPU(int, sd_llc_id) = -1;
>>>>>   DEFINE_PER_CPU(int, sd_share_id);
>>>>>   DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
>>>>>   DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa);
>>>>> @@ -684,7 +685,6 @@ static void update_top_cache_domain(int cpu)
>>>>>         rcu_assign_pointer(per_cpu(sd_llc, cpu), sd);
>>>>>       per_cpu(sd_llc_size, cpu) = size;
>>>>> -    per_cpu(sd_llc_id, cpu) = id;
>>>>>       rcu_assign_pointer(per_cpu(sd_llc_shared, cpu), sds);
>>>>>         sd = lowest_flag_domain(cpu, SD_CLUSTER);
>>>>> @@ -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)
>>
>> 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 (!!!)
>>
>>
>> Instead, if we traversed to tl_mc, we would have seen all the online
>> CPUs in the MC and reused the llc_id from them. Might not be an issue on
>> its own but if this root partition is removed later, CPU8 will continue
>> to have the unique llc_id even after merging into the same MC domain.
> 
> There is really no reason to reuse the llc_id as far as cache aware scheduling
> goes in its v3 revision (see my reply to Madadi on this patch).  

Even I don't mind having some holes in the llc_id space when CPUs are
offlined but my major concern would be seeing an inconsistent state
where CPUs in same MC domains end up with different llc_id when after
a bunch of hotplug activity.

>  
> I am thinking that if we just simply rebuild LLC id across sched domain
> rebuilds, that is probably the cleanest solution. There could be some races
> in cpus_share_cache() as llc_id gets reassigned for some CPUs when they
> come online/offline.  But we also having similar races in current mainline code.
> Worst it can do is some temporary sub-optimal scheduling task placement. 
> 
> Thoughts?

If you are suggesting populating the sd_llc_id for all the CPUs on
topology rebuild, I'm not entirely against the idea.

On a separate note, if we add a dependency on SCHED_MC for SCHED_CACHE,
we can simply look at cpu_coregroup_mask() and either allocate a new
llc_id / borrow llc id  in sched_cpu_activate() when CPU is onlined or
reassign them in sched_cpu_deactivate() if an entire LLC is offlined.

-- 
Thanks and Regards,
Prateek