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

[Chen, Yu C]

On 2/21/2026 12:03 AM, Peter Zijlstra wrote:
> On Fri, Feb 20, 2026 at 11:53:31PM +0800, Chen, Yu C wrote:
> > Hi Peter,
> >
> > On 2/19/2026 11:40 PM, Peter Zijlstra wrote:
> > > On Mon, Feb 16, 2026 at 01:14:20PM +0530, K Prateek Nayak wrote:
> >
> > [ ... ]
> >
> > > > +static int __sched_domains_alloc_llc_id(void)
> > > > +{
> > > > +	int lid;
> > > > +
> > > > +	lockdep_assert_held(&sched_domains_mutex);
> > > > +
> > > > +	lid = cpumask_first_zero(sched_domains_llc_id_allocmask);
> > > > +	if (lid >= tl_max_llcs)
> > > > +		tl_max_llcs++;
> > >
> > > Urgh,. should we not rather track the max lid?
> >
> > Do you mean we should not always increment the max lid,
> > but instead decrease it when an llc_id is released?
> > I think Tim has adjusted the code to shrink tl_max_llcs
> > when an llc_id is released:
> > https://lore.kernel.org/all/acc7a5c96e8235bf11af640798ce1b60bcaa8196.camel@xxxxxxxxxxxxxxx/
>
> You can only shrink when the max lid is released. Since lid is an array
> index, something like max_lid = weight(mask) would be terribly broken.
>
> But what I was getting at is that the code as presented there is rather
> non-obvious. Yes, if the lid is higher, it cannot be more than one
> higher than the previous value, but something like:
>
> 	lid = cpumask_first_zero();
> 	BUG_ON(lid >= nr_cpu_ids);
> 	max_lid = max(max_lid, lid);
>
> Is way simpler to follow since it doesn't have that hidden assumption.
>
> Then, if you want to allow shrinking, then the clear side could do
> something like:
>
> 	__cpumask_clear(lid, mask);
> 	if (lid == max_lid)
> 		max_lid = cpumask_last(mask);
>
> or something like that.

Got it, we will adjust the code accordingly.

thanks,
Chenyu