Subject: Re: [PATCH v2 1/2] sched/topology: Skip updating masks for non-online nodes

From: Srikar Dronamraju <[email-masked]>
Date: Mon, 12 Jul 2021 18:18:56 +0530

Hi Valentin,

> On 01/07/21 09:45, Srikar Dronamraju wrote:
> > @@ -1891,12 +1894,30 @@ void sched_init_numa(void)
> >  void sched_domains_numa_masks_set(unsigned int cpu)
> >  {
> 
> Hmph, so we're playing games with masks of offline nodes - is that really
> necessary? Your modification of sched_init_numa() still scans all of the
> nodes (regardless of their online status) to build the distance map, and
> that is never updated (sched_init_numa() is pretty much an __init
> function).
> 
> So AFAICT this is all to cope with topology_span_sane() not applying
> 'cpu_map' to its masks. That seemed fine to me back when I wrote it, but in
> light of having bogus distance values for offline nodes, not so much...
> 
> What about the below instead?
> 
> ---
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index b77ad49dc14f..c2d9caad4aa6 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2075,6 +2075,7 @@ static struct sched_domain *build_sched_domain(struct sched_domain_topology_leve
>  static bool topology_span_sane(struct sched_domain_topology_level *tl,
>  			      const struct cpumask *cpu_map, int cpu)
>  {
> +	struct cpumask *intersect = sched_domains_tmpmask;
>  	int i;
> 
>  	/* NUMA levels are allowed to overlap */
> @@ -2090,14 +2091,17 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
>  	for_each_cpu(i, cpu_map) {
>  		if (i == cpu)
>  			continue;
> +
>  		/*
> -		 * We should 'and' all those masks with 'cpu_map' to exactly
> -		 * match the topology we're about to build, but that can only
> -		 * remove CPUs, which only lessens our ability to detect
> -		 * overlaps
> +		 * We shouldn't have to bother with cpu_map here, unfortunately
> +		 * some architectures (powerpc says hello) have to deal with
> +		 * offline NUMA nodes reporting bogus distance values. This can
> +		 * lead to funky NODE domain spans, but since those are offline
> +		 * we can mask them out.
>  		 */
> +		cpumask_and(intersect, tl->mask(cpu), tl->mask(i));
>  		if (!cpumask_equal(tl->mask(cpu), tl->mask(i)) &&
> -		    cpumask_intersects(tl->mask(cpu), tl->mask(i)))
> +		    cpumask_intersects(intersect, cpu_map))
>  			return false;
>  	}
> 

Unfortunately this is not helping.
I tried this patch alone and also with 2/2 patch of this series where
we update/fill fake topology numbers. However both cases are still failing.

-- 
Thanks and Regards
Srikar Dronamraju