Re: [PATCH v5] sched/fair: Use sched_domain_span() for topology_span_sane()

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Peter,

On 8/25/2025 2:49 PM, Peter Zijlstra wrote:
> We should either set ->mask to NULL for NUMA thingies to make sure we
> don't end up using it again, or bite the bullet and fix up the mask
> function declaration.
> 
> Something a little like so?

Both the QEMU VM from the above commit message and my 3rd Generation
EPYC are happy with the changes. One concern:

> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 977e133bb8a4..8164ffabcd31 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1732,22 +1724,42 @@ sd_init(struct sched_domain_topology_level *tl,
>  	return sd;
>  }
>  
> +static const struct cpumask *tl_smt_mask(struct sched_domain_topology_level *tl, int cpu)
> +{
> +	return cpu_smt_mask(cpu);
> +}
> +
> +static const struct cpumask *tl_cls_mask(struct sched_domain_topology_level *tl, int cpu)
> +{
> +	return q(cpu);
> +}
> +
> +static const struct cpumask *tl_mc_mask(struct sched_domain_topology_level *tl, int cpu)
> +{
> +	return cpu_coregroup_mask(cpu);
> +}
> +

The above helpers may need guarding behind CONFIG_SCHED_{SMT,CLUSTER,MC}
if I'm not mistaken. Possibility for some unification and cleanup with:

(Only build and boot tested on x86 on top of tip/master + your diff)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 0b5897fff687..a0eeb6e39304 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1054,16 +1054,6 @@ static bool has_coregroup_support(void)
 	return coregroup_enabled;
 }
 
-static const struct cpumask *cpu_mc_mask(struct sched_domain_topology_level *tl, int cpu)
-{
-	return cpu_coregroup_mask(cpu);
-}
-
-static const struct cpumask *cpu_pkg_mask(struct sched_domain_topology_level *tl, int cpu)
-{
-	return cpu_node_mask(cpu);
-}
-
 static int __init init_big_cores(void)
 {
 	int cpu;
@@ -1718,10 +1708,10 @@ static void __init build_sched_topology(void)
 
 	if (has_coregroup_support()) {
 		powerpc_topology[i++] =
-			SDTL_INIT(cpu_mc_mask, powerpc_shared_proc_flags, MC);
+			SDTL_INIT(tl_mc_mask, powerpc_shared_proc_flags, MC);
 	}
 
-	powerpc_topology[i++] = SDTL_INIT(cpu_pkg_mask, powerpc_shared_proc_flags, PKG);
+	powerpc_topology[i++] = SDTL_INIT(tl_pkg_mask, powerpc_shared_proc_flags, PKG);
 
 	/* There must be one trailing NULL entry left.  */
 	BUG_ON(i >= ARRAY_SIZE(powerpc_topology) - 1);
diff --git a/arch/s390/kernel/topology.c b/arch/s390/kernel/topology.c
index df036ab83920..68d22cf3c604 100644
--- a/arch/s390/kernel/topology.c
+++ b/arch/s390/kernel/topology.c
@@ -530,17 +530,12 @@ static const struct cpumask *cpu_drawer_mask(struct sched_domain_topology_level
 	return &cpu_topology[cpu].drawer_mask;
 }
 
-static const struct cpumask *cpu_pkg_mask(struct sched_domain_topology_level *tl, int cpu)
-{
-	return cpu_node_mask(cpu);
-}
-
 static struct sched_domain_topology_level s390_topology[] = {
 	SDTL_INIT(cpu_thread_mask, cpu_smt_flags, SMT),
 	SDTL_INIT(cpu_coregroup_mask, cpu_core_flags, MC),
 	SDTL_INIT(cpu_book_mask, NULL, BOOK),
 	SDTL_INIT(cpu_drawer_mask, NULL, DRAWER),
-	SDTL_INIT(cpu_pkg_mask, NULL, PKG),
+	SDTL_INIT(tl_pkg_mask, NULL, PKG),
 	{ NULL, },
 };
 
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 31a3b57314ef..eb289abece23 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -471,26 +471,6 @@ static int x86_cluster_flags(void)
 }
 #endif
 
-static const struct cpumask *tl_smt_mask(struct sched_domain_topology_level *tl, int cpu)
-{
-	return cpu_smt_mask(cpu);
-}
-
-static const struct cpumask *tl_cls_mask(struct sched_domain_topology_level *tl, int cpu)
-{
-	return cpu_clustergroup_mask(cpu);
-}
-
-static const struct cpumask *tl_mc_mask(struct sched_domain_topology_level *tl, int cpu)
-{
-	return cpu_coregroup_mask(cpu);
-}
-
-static const struct cpumask *tl_pkg_mask(struct sched_domain_topology_level *tl, int cpu)
-{
-	return cpu_node_mask(cpu);
-}
-
 /*
  * Set if a package/die has multiple NUMA nodes inside.
  * AMD Magny-Cours, Intel Cluster-on-Die, and Intel
diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index f0a53b0e67f5..c7457ccf05c4 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -30,11 +30,19 @@ struct sd_flag_debug {
 };
 extern const struct sd_flag_debug sd_flag_debug[];
 
+struct sched_domain_topology_level;
+
 #ifdef CONFIG_SCHED_SMT
 static inline int cpu_smt_flags(void)
 {
 	return SD_SHARE_CPUCAPACITY | SD_SHARE_LLC;
 }
+
+static const __maybe_unused
+struct cpumask *tl_smt_mask(struct sched_domain_topology_level *tl, int cpu)
+{
+	return cpu_smt_mask(cpu);
+}
 #endif
 
 #ifdef CONFIG_SCHED_CLUSTER
@@ -42,6 +50,12 @@ static inline int cpu_cluster_flags(void)
 {
 	return SD_CLUSTER | SD_SHARE_LLC;
 }
+
+static const __maybe_unused
+struct cpumask *tl_cls_mask(struct sched_domain_topology_level *tl, int cpu)
+{
+	return cpu_clustergroup_mask(cpu);
+}
 #endif
 
 #ifdef CONFIG_SCHED_MC
@@ -49,8 +63,20 @@ static inline int cpu_core_flags(void)
 {
 	return SD_SHARE_LLC;
 }
+
+static const __maybe_unused
+struct cpumask *tl_mc_mask(struct sched_domain_topology_level *tl, int cpu)
+{
+	return cpu_coregroup_mask(cpu);
+}
 #endif
 
+static const __maybe_unused
+struct cpumask *tl_pkg_mask(struct sched_domain_topology_level *tl, int cpu)
+{
+	return cpu_node_mask(cpu);
+}
+
 #ifdef CONFIG_NUMA
 static inline int cpu_numa_flags(void)
 {
@@ -172,8 +198,6 @@ bool cpus_equal_capacity(int this_cpu, int that_cpu);
 bool cpus_share_cache(int this_cpu, int that_cpu);
 bool cpus_share_resources(int this_cpu, int that_cpu);
 
-struct sched_domain_topology_level;
-
 typedef const struct cpumask *(*sched_domain_mask_f)(struct sched_domain_topology_level *tl, int cpu);
 typedef int (*sched_domain_flags_f)(void);
 
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 8164ffabcd31..18889bd97e22 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1724,26 +1724,6 @@ sd_init(struct sched_domain_topology_level *tl,
 	return sd;
 }
 
-static const struct cpumask *tl_smt_mask(struct sched_domain_topology_level *tl, int cpu)
-{
-	return cpu_smt_mask(cpu);
-}
-
-static const struct cpumask *tl_cls_mask(struct sched_domain_topology_level *tl, int cpu)
-{
-	return cpu_clustergroup_mask(cpu);
-}
-
-static const struct cpumask *tl_mc_mask(struct sched_domain_topology_level *tl, int cpu)
-{
-	return cpu_coregroup_mask(cpu);
-}
-
-static const struct cpumask *tl_pkg_mask(struct sched_domain_topology_level *tl, int cpu)
-{
-	return cpu_node_mask(cpu);
-}
-
 /*
  * Topology list, bottom-up.
  */
--

Thoughts?

-- 
Thanks and Regards,
Prateek