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

[Chen, Yu C]

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@xxxxxxxxx>

[...]

> > > > @@ -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