linux-kernel - Re: [PATCH] sched/topology: Check average distances to remote packages

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Message-ID: <aYPRGsMaicbWglFy@hpe.com>
Date: Wed, 4 Feb 2026 17:07:06 -0600
From: Kyle Meyer <kyle.meyer@....com>
To: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: bp@...en8.de, dave.hansen@...ux.intel.com, mingo@...hat.com,
        peterz@...radead.org, tglx@...nel.org, vinicius.gomes@...el.com,
        brgerst@...il.com, hpa@...or.com, kprateek.nayak@....com,
        linux-kernel@...r.kernel.org, patryk.wlazlyn@...ux.intel.com,
        rafael.j.wysocki@...el.com, russ.anderson@....com, x86@...nel.org,
        yu.c.chen@...el.com, zhao1.liu@...el.com
Subject: Re: [PATCH] sched/topology: Check average distances to remote
 packages

On Wed, Feb 04, 2026 at 02:58:46PM -0800, Tim Chen wrote:
> On Tue, 2026-01-27 at 11:02 -0600, Kyle Meyer wrote:
> > Granite Rapids (GNR) and Clearwater Forest (CWF) average distances to
> > remote packages to fix scheduler domains, see [1] for more information.
> > 
> > A warning and backtrace are printed when sub-NUMA clustering (SNC) is
> > enabled and there are more than 2 packages because the average distances
> > to remote packages could be different, skewing the single average remote
> > distance.
> > 
> > This is unnecessary when the average distances to remote packages are
> > the same.
> > 
> > Support single average remote distance on systems with more than 2
> > packages, preventing unnecessary warnings and backtraces by checking if
> > average distances to remote packages are the same.
> > 
> > [1] commit 4d6dd05d07d0 ("sched/topology: Fix sched domain build error for GNR, CWF in SNC-3 mode").
> > 
> > Signed-off-by: Kyle Meyer <kyle.meyer@....com>
> > ---
> > 
> > The warning and backtrace were noticed on a 16 socket GNR system with SNC-2 enabled.
> > 
> > ---
> >  arch/x86/kernel/smpboot.c | 70 ++++++++++++++++++++++++++++-----------
> >  1 file changed, 51 insertions(+), 19 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > index 5cd6950ab672..4467716f4054 100644
> > --- a/arch/x86/kernel/smpboot.c
> > +++ b/arch/x86/kernel/smpboot.c
> > @@ -518,27 +518,70 @@ static int avg_remote_numa_distance(void)
> >  {
> >  	int i, j;
> >  	int distance, nr_remote, total_distance;
> > +	int max_pkgs = topology_max_packages();
> > +	int cpu, pkg, pkg_avg_distance;
> > +	int *pkg_total_distance;
> > +	int *pkg_nr_remote;
> 
> Minor nit, pkg_nr_remote needs to be initialized as NULL.
> Otherwise if allocation of pkg_total_distance failed and we
> go to cleanup, we can call kfree(pkg_nr_remote) with a non
> null uninitialized pkg_nr_remote pointer.  Maybe do
> 
> 	int *pkg_total_distance=NULL, *pkg_nr_remote=NULL;
> 
> in declaration.

Thanks for the review! I'll send a v2.

> >  	if (sched_avg_remote_distance > 0)
> >  		return sched_avg_remote_distance;
> >  
> > +	sched_avg_remote_distance = REMOTE_DISTANCE;
> > +
> >  	nr_remote = 0;
> >  	total_distance = 0;
> > +
> > +	pkg_total_distance = kcalloc(max_pkgs, sizeof(int), GFP_KERNEL);
> > +	if (!pkg_total_distance)
> > +		goto cleanup;
> > +
> > +	pkg_nr_remote = kcalloc(max_pkgs, sizeof(int), GFP_KERNEL);
> > +	if (!pkg_nr_remote)
> > +		goto cleanup;
> > +
> >  	for_each_node_state(i, N_CPU) {
> >  		for_each_node_state(j, N_CPU) {
> >  			distance = node_distance(i, j);
> >  
> > -			if (distance >= REMOTE_DISTANCE) {
> > -				nr_remote++;
> > -				total_distance += distance;
> > -			}
> > +			if (distance < REMOTE_DISTANCE)
> > +				continue;
> > +
> > +			nr_remote++;
> > +			total_distance += distance;
> > +
> > +			cpu = cpumask_first(cpumask_of_node(j));
> > +			if (cpu >= nr_cpu_ids)
> > +				continue;
> > +
> > +			pkg = topology_physical_package_id(cpu);
> > +			pkg_total_distance[pkg] += distance;
> > +			pkg_nr_remote[pkg]++;
> >  		}
> >  	}
> > -	if (nr_remote)
> > -		sched_avg_remote_distance = total_distance / nr_remote;
> > -	else
> > -		sched_avg_remote_distance = REMOTE_DISTANCE;
> >  
> > +	if (!nr_remote)
> > +		goto cleanup;
> > +
> > +	sched_avg_remote_distance = total_distance / nr_remote;
> > +
> > +	/*
> > +	 * Single average remote distance won't be appropriate if different
> > +	 * packages have different distances to remote packages.
> > +	 */
> > +	for (i = 0; i < max_pkgs; i++) {
> > +		if (!pkg_nr_remote[i])
> > +			continue;
> > +
> > +		pkg_avg_distance = pkg_total_distance[i] / pkg_nr_remote[i];
> > +
> > +		pr_debug("sched: Avg. distance to remote package %d: %d\n", i, pkg_avg_distance);
> > +
> > +		if (pkg_avg_distance != sched_avg_remote_distance)
> > +			WARN_ONCE(1, "sched: Avg. distances to remote packages are different\n");
> > +	}
> > +cleanup:
> > +	kfree(pkg_nr_remote);
> > +	kfree(pkg_total_distance);
> >  	return sched_avg_remote_distance;
> >  }
> >  
> > @@ -564,18 +607,7 @@ int arch_sched_node_distance(int from, int to)
> >  		 * in the remote package in the same sched group.
> >  		 * Simplify NUMA domains and avoid extra NUMA levels including
> >  		 * different remote NUMA nodes and local nodes.
> > -		 *
> > -		 * GNR and CWF don't expect systems with more than 2 packages
> > -		 * and more than 2 hops between packages. Single average remote
> > -		 * distance won't be appropriate if there are more than 2
> > -		 * packages as average distance to different remote packages
> > -		 * could be different.
> >  		 */
> > -		WARN_ONCE(topology_max_packages() > 2,
> > -			  "sched: Expect only up to 2 packages for GNR or CWF, "
> > -			  "but saw %d packages when building sched domains.",
> > -			  topology_max_packages());
> > -
> >  		d = avg_remote_numa_distance();
> >  	}
> >  	return d;