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

[Chen, Yu C]

On 2/18/2026 11:28 AM, K Prateek Nayak wrote:
> 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. 

Tim, do you mean reset all CPUs' LLC id to -1 whenever there is hotplug
event in partition_sched_domains_locked(), and rebuild them from scratch
in build_sched_domains(), so we already refresh the LLC id for every
CPU(I discussed with Vineeth here:
https://lore.kernel.org/all/54e60704-b0f3-44df-9b83-070806b5a00c@xxxxxxxxx/)


> > 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.

Prateek, may I know if you are thinking of updating every CPU's LLC id
during its hotplug and not update all percpu LLC id in 
build_sched_domains()?

thanks,
Chenyu