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

[Andrea Righi]

Hi Prateek,

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.

Ah, good catch! When cpumask_first(asym_span) == cpumask_first(llc_span)
(big.LITTLE typical case), both sd_asym->shared and sd_llc->shared would alias
to d->sds[0].

> 
> 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)
> 
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index fe09d3268bc9..1d2c98dca211 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -67,7 +67,15 @@ struct sched_domain_shared {
>  	atomic_t	ref;
>  	atomic_t	nr_busy_cpus;
>  	int		has_idle_cores;
> -	int		nr_idle_scan;
> +	union {
> +		int	nr_idle_scan;
> +		/*
> +		 * Used during allocation to claim the
> +		 * sched_domain_shared object at
> +		 * multiple levels.

I think between build and the first LB tick, readers of nr_idle_scan may observe
leftover SD_* flags in nr_idle_scan. This shouldn't be a problem and should
self-heal soon, but maybe it's worth a comment? Something like:

  * Note: between build and the first periodic LB tick, which
  * rewrites the union via update_idle_cpu_scan(), readers of
  * nr_idle_scan may observe the transient SD_* flag value as
  * the scan bound. The flag bits are small positive integers,
  * so the effect is just a slightly relaxed scan bound for one
  * window and self-heals on the first tick.

> +		 */
> +		int	alloc_flags;
> +	};
>  #ifdef CONFIG_SCHED_CACHE
>  	unsigned long	util_avg;
>  	unsigned long	capacity;
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index dbfd9657f897..9ebd14652e9d 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -623,6 +623,12 @@ static void free_sched_groups(struct sched_group *sg, int free_sgc)
>  	} while (sg != first);
>  }
>  
> +static void free_sched_domain_shared(struct sched_domain_shared *sds)
> +{
> +	if (sds && atomic_dec_and_test(&sds->ref))
> +		kfree(sds);
> +}
> +
>  static void destroy_sched_domain(struct sched_domain *sd)
>  {
>  	/*
> @@ -631,9 +637,7 @@ static void destroy_sched_domain(struct sched_domain *sd)
>  	 * dropping group/capacity references, freeing where none remain.
>  	 */
>  	free_sched_groups(sd->groups, 1);
> -
> -	if (sd->shared && atomic_dec_and_test(&sd->shared->ref))
> -		kfree(sd->shared);
> +	free_sched_domain_shared(sd->shared);
>  
>  #ifdef CONFIG_SCHED_CACHE
>  	/* only the bottom sd has llc_counts array */
> @@ -755,7 +759,14 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
>  
>  			/* Pick reference to parent->shared. */
>  			if (parent->shared) {
> -				WARN_ON_ONCE(tmp->shared);
> +				/*
> +				 * It is safe to free a sd->shared that
> +				 * has not been published yet. If a
> +				 * sd->shared was published, the refcount
> +				 * will end up being non-zero and it will
> +				 * not be freed here.
> +				 */
> +				free_sched_domain_shared(tmp->shared);
>  				tmp->shared = parent->shared;
>  				parent->shared = NULL;
>  			}
> @@ -2916,11 +2927,40 @@ static void adjust_numa_imbalance(struct sched_domain *sd_llc)
>  	}
>  }
>  
> -static void init_sched_domain_shared(struct s_data *d, struct sched_domain *sd)
> +static void
> +init_sched_domain_shared(struct s_data *d, struct sched_domain *sd, int flags)
>  {
> -	int sd_id = cpumask_first(sched_domain_span(sd));
> +	int cpu;
> +
> +	/*
> +	 * Multiple domains can try to claim a shared object like
> +	 * SD_ASYM_CPUCAPACITY and SD_SHARE_LLC which can alias to
> +	 * same cpumask_first(sched_domain_span(sd)) CPU and can
> +	 * cause "nr_idle_scan" to be populated incorrectly during
> +	 * load balncing.

nit: s/balncing/balancing/

> +	 *
> +	 * Find the first CPU in sched_domain_span(sd) with an
> +	 * unclaimed domain (!alloc_flags) or where the alloc_flag
> +	 * matches the requested flag (SD_* flag)
> +	 */
> +	for_each_cpu(cpu, sched_domain_span(sd)) {
> +		struct sched_domain_shared *sds = *per_cpu_ptr(d->sds, cpu);
> +
> +		/*
> +		 * If the domain only has single CPU, allow temporary overlap
> +		 * in allocation since the domains will be degenerated anyways.
> +		 */
> +		if (!sds->alloc_flags ||
> +		    sd->span_weight == 1 ||
> +		    sds->alloc_flags == flags) {
> +			sds->alloc_flags = flags;
> +			sd->shared = sds;
> +			break;
> +		}
> +	}
> +
> +	BUG_ON(!sd->shared);

Unreachable in practice, but should we have a WARN_ON_ONCE() +
bail/early-return? In this way we'd fall back to using LLC's shared for
sd_balance_shared, which seems nicer than a BUG_ON().

>  
> -	sd->shared = *per_cpu_ptr(d->sds, sd_id);
>  	/*
>  	 * nr_busy_cpus is consumed only by the NOHZ kick path via
>  	 * sd_balance_shared; on the asym-capacity path it is initialized but
> @@ -2960,7 +3000,7 @@ static bool claim_asym_sched_domain_shared(struct s_data *d, int cpu)
>  	if (!sd_asym || (sd_asym->flags & SD_NUMA))
>  		return false;
>  
> -	init_sched_domain_shared(d, sd_asym);
> +	init_sched_domain_shared(d, sd_asym, SD_ASYM_CPUCAPACITY);
>  	return true;
>  }
>  
> @@ -3115,7 +3155,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>  			sd = sd->parent;
>  
>  		if (sd->flags & SD_SHARE_LLC) {
> -			init_sched_domain_shared(&d, sd);
> +			init_sched_domain_shared(&d, sd, SD_SHARE_LLC);
>  
>  			/*
>  			 * In presence of higher domains, adjust the
> -- 
> Thanks and Regards,
> Prateek
> 

Thanks,
-Andrea