linux-kernel - Re: [PATCH v3 1/2] sched: Create architecture specific sched domain distances

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Message-ID: <abf5742091e290e26f49a0a7a28f54f19085e334.camel@linux.intel.com>
Date: Mon, 15 Sep 2025 10:16:05 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: "Chen, Yu C" <yu.c.chen@...el.com>, Peter Zijlstra
 <peterz@...radead.org>,  Ingo Molnar <mingo@...hat.com>
Cc: Juri Lelli <juri.lelli@...hat.com>, Dietmar Eggemann	
 <dietmar.eggemann@....com>, Ben Segall <bsegall@...gle.com>, Mel Gorman	
 <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, Tim Chen	
 <tim.c.chen@...el.com>, Vincent Guittot <vincent.guittot@...aro.org>, Libo
 Chen	 <libo.chen@...cle.com>, Abel Wu <wuyun.abel@...edance.com>, Len Brown
	 <len.brown@...el.com>, linux-kernel@...r.kernel.org, K Prateek Nayak	
 <kprateek.nayak@....com>, "Gautham R . Shenoy" <gautham.shenoy@....com>, 
 Zhao Liu <zhao1.liu@...el.com>, Vinicius Costa Gomes
 <vinicius.gomes@...el.com>, Arjan Van De Ven	 <arjan.van.de.ven@...el.com>
Subject: Re: [PATCH v3 1/2] sched: Create architecture specific sched domain
 distances

On Fri, 2025-09-12 at 13:24 +0800, Chen, Yu C wrote:
> On 9/12/2025 2:30 AM, Tim Chen wrote:
> > Allow architecture specific sched domain NUMA distances that can be
> > modified from NUMA node distances for the purpose of building NUMA
> > sched domains.
> > 
> > The actual NUMA distances are kept separately.  This allows for NUMA
> > domain levels modification when building sched domains for specific
> > architectures.
> > 
> > Consolidate the recording of unique NUMA distances in an array to
> > sched_record_numa_dist() so the function can be reused to record NUMA
> > distances when the NUMA distance metric is changed.
> > 
> > No functional change if there's no arch specific NUMA distances
> > are being defined.
> > 
> 
> [snip]
> 
> > +
> > +void sched_init_numa(int offline_node)
> > +{
> > +	struct sched_domain_topology_level *tl;
> > +	int nr_levels, nr_node_levels;
> > +	int i, j;
> > +	int *distances, *domain_distances;
> > +	struct cpumask ***masks;
> > +
> > +	if (sched_record_numa_dist(offline_node, numa_node_dist, &distances,
> > +				   &nr_node_levels))
> > +		return;
> > +
> > +	WRITE_ONCE(sched_avg_remote_numa_distance,
> > +		   avg_remote_numa_distance(offline_node));
> > +
> > +	if (sched_record_numa_dist(offline_node,
> > +				   arch_sched_node_distance, &domain_distances,
> > +				   &nr_levels)) {
> > +		kfree(distances);
> > +		return;
> > +	}
> > +	rcu_assign_pointer(sched_numa_node_distance, distances);
> > +	WRITE_ONCE(sched_max_numa_distance, distances[nr_node_levels - 1]);
> 
> [snip]
> 
> > @@ -2022,7 +2097,6 @@ void sched_init_numa(int offline_node)
> >   	sched_domain_topology = tl;
> >   
> >   	sched_domains_numa_levels = nr_levels;
> > -	WRITE_ONCE(sched_max_numa_distance, sched_domains_numa_distance[nr_levels - 1]);
> >   
> 
> Before this patch, sched_max_numa_distance is assigned a valid
> value at the end of sched_init_numa(), after sched_domains_numa_masks
>   and sched_domain_topology_level are successfully created or appended
> , the kzalloc() call should succeed.
> 
> Now we assign sched_max_numa_distance earlier, without considering
> the status of NUMA sched domains. I think this is intended, because
>   sched domains are only for generic load balancing, while
>   sched_max_numa_distance is for NUMA load balancing; in theory, they
> use different metrics in their strategies. Thus, this change should
> not cause any issues.
> 
>  From my understanding,
> 
> Reviewed-by: Chen Yu <yu.c.chen@...el.com>
> 
> thanks,
> Chenyu