Re: [PATCH v2 2/5] sched/fair: Attach sched_domain_shared to sd_asym_cpucapacity

[Peter Zijlstra]

On Tue, May 19, 2026 at 11:22:32AM +0530, K Prateek Nayak wrote:
> Hello Peter, Andrea,
> 
> On 5/19/2026 2:28 AM, Peter Zijlstra wrote:
> > @@@ -2775,20 -3049,16 +3107,15 @@@ build_sched_domains(const struct cpumas
> >   		if (!sd)
> >   			continue;
> >   
> >  +		if (has_asym)
> > - 			asym_claimed = claim_asym_sched_domain_shared(&d, i);
> > ++			claim_asym_sched_domain_shared(&d, i);
> >  +
> >   		/* First, find the topmost SD_SHARE_LLC domain */
> >   		while (sd->parent && (sd->parent->flags & SD_SHARE_LLC))
> >   			sd = sd->parent;
> >   
> >   		if (sd->flags & SD_SHARE_LLC) {
> > - 			/*
> > - 			 * Initialize the sd->shared for SD_SHARE_LLC unless
> > - 			 * the asym path above already claimed it.
> > - 			 */
> > - 			if (!asym_claimed)
> > - 				init_sched_domain_shared(&d, sd);
> >  -			int sd_id = cpumask_first(sched_domain_span(sd));
> >  -
> >  -			sd->shared = *per_cpu_ptr(d.sds, sd_id);
> >  -			atomic_set(&sd->shared->nr_busy_cpus, sd->span_weight);
> >  -			atomic_inc(&sd->shared->ref);
> > ++			init_sched_domain_shared(&d, sd);
> 
> This will run into a small problem with "nr_idle_scan" if
> cpumask_first(sched_domain_span(sd)) is the same for both sd_asym and
> sd_llc.
> 
> Load balancer at different domains will populate "nr_idle_scan" with
> different values and they alias to same ->shared if one isn't
> degenerated and I believe there is at least one way to hit the WARN_ON()
> from cpu_attach_domain() if the SD_ASYM_CPUCAPACITY_FULL comes before
> the last SD_SHARE_LLC domain and the latter is degenerated.
> 
> How about this:
> 
>   (On top of queue:sched/core; Lightly tested on !ASYM_CPUCAPACITY system)

Shrikanth noted I had an old version of his SMT ifdeffery patches on, so
I need to rebuild that tree (and the merge) anyway.

Do you want me to munge this in, or keep it on top as a fixie?