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

[Chen, Yu C]

On 2/20/2026 5:04 AM, Tim Chen wrote:
> On Thu, 2026-02-19 at 11:20 -0800, Tim Chen wrote:
> > On Thu, 2026-02-19 at 23:20 +0800, Chen, Yu C wrote:
> > > On 2/19/2026 10:59 PM, Peter Zijlstra wrote:
> > > > On Tue, Feb 10, 2026 at 02:18:44PM -0800, Tim Chen wrote:

[ ... ]

> > > > > +
> > > > > +		lid = per_cpu(sd_llc_id, i);
> > > > > +		if (lid == -1) {
> > > > > +			int j;
> > > > > +
> > > > > +			/*
> > > > > +			 * Assign the llc_id to the CPUs that do not
> > > > > +			 * have an LLC.
> > > > > +			 */
> > > >
> > > > Where does this happen? Is this for things like Atom that don't have an
> > > > L3 and so we don't set up a LLC domain?
> > >
> > > Yes, for some hybrid platforms, some CPUs on that platforms might not
> > > have L3,
> > > Tim might correct me if I’m wrong. Above code is derived from the
> > > update_top_cache_domain(),
> > >    if there is no sched domain with SD_SHARE_LLC, per_cpu(sd_llc_id, cpu)
> > > is set to the
> > > CPU number directly.
> >
> > That's correct.  One example is Meteor Lake where some Atom CPUs don't have
> > L3 but have only L2.  And some Ampere CPUs also have no shared L3.
> >
> > https://www.spinics.net/lists/kernel/msg5863118.html?utm_source=chatgpt.com
> >
> > This also reminded me that if we rely on cpu_coregroup_mask for LLC id
> > assignment, we may be missing out such platforms which need to treat
> > L2 as the last level cache.  So we may need to fallback to cpu_clustergroup_mask
> > or cpu_smt_mask where applicable.
>
> On further inspection of the code, cpu_coregroup_mask will just be the same
> as cpu_clustergroup_mask for that case so we should be okay.

OK, I assume this is true for Intel platforms because the llc_id will
be set to l2_id if there is no L3 cache:
c->topo.llc_id = (l3_id == BAD_APICID) ? l2_id : l3_id;
I suppose AMD platforms should not be impacted because I have not seen
any non-L3 platforms (for AMD).
For non-x86 platforms, cpu_coregroup_mask() will be converted to the
cluster mask if no LLC is present.

thanks,
Chenyu