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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 17, 2026 at 01:39:45PM +0530, K Prateek Nayak wrote:
> 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 (!!!)

Yeah, so partitions (including isol_cpus) could wreck things here, since
this is purely about the sched_domains.

You can create N single CPU partitions (isol_cpus does this) and end up
with the same 'problem' that online one at a time loop did. Except this
time it would not be 'wrong'. Since they are single CPU domains, you
also don't get load-balancing, so who cares I suppose. But it will
inflate max_lid.

But suppose you create N/2 partitions (where N is the number of CPUs in
the physical LLC), then you get many individual 'LLC's and
load-balancing inside them. I suppose this is correct, although it does
inflate max_lid somewhat beyond what you would normally expect.

However, most of that space would be wasted, since you're not actually
allowed to migrate to them.