[PATCH v2] arm: Fix topology setup in case of CPU hotplug for CONFIG_SCHED_MC

Dietmar Eggemann dietmar.eggemann at arm.com
Wed Nov 27 21:33:53 AEDT 2019

Commit ca74b316df96 ("arm: Use common cpu_topology structure and
functions.") changed cpu_coregroup_mask() from the ARM32 specific
implementation in arch/arm/include/asm/topology.h to the one shared
with ARM64 and RISCV in drivers/base/arch_topology.c.

Currently on ARM32 (TC2 w/ CONFIG_SCHED_MC) the task scheduler setup
code (w/ CONFIG_SCHED_DEBUG) shows this during CPU hotplug:

  ERROR: groups don't span domain->span

It happens to CPUs of the cluster of the CPU which gets hot-plugged
out on scheduler domain MC.

Turns out that the shared cpu_coregroup_mask() requires that the
hot-plugged CPU is removed from the core_sibling mask via
remove_cpu_topology(). Otherwise the 'is core_sibling subset of
cpumask_of_node()' doesn't work. In this case the task scheduler has to
deal with cpumask_of_node instead of core_sibling which is wrong on
scheduler domain MC.

e.g. CPU3 hot-plugged out on TC2 [cluster0: 0,3-4 cluster1: 1-2]:

  cpu_coregroup_mask(): CPU3 cpumask_of_node=0-2,4 core_sibling=0,3-4
should be:

  cpu_coregroup_mask(): CPU3 cpumask_of_node=0-2,4 core_sibling=0,4

Add remove_cpu_topology() to __cpu_disable() to remove the CPU from the
topology masks in case of a CPU hotplug out operation.

At the same time tweak store_cpu_topology() slightly so it will call
update_siblings_masks() in case of CPU hotplug in operation via

This aligns the ARM32 implementation with the ARM64 one.

Guarding remove_cpu_topology() with CONFIG_GENERIC_ARCH_TOPOLOGY is
necessary since some Arm32 defconfigs (aspeed_g5_defconfig,
milbeaut_m10v_defconfig, spear13xx_defconfig) specify an explicit


w/ ./arch/arm/Kconfig: select GENERIC_ARCH_TOPOLOGY if ARM_CPU_TOPOLOGY

Fixes: ca74b316df96 ("arm: Use common cpu_topology structure and functions")
Reviewed-by: Sudeep Holla <sudeep.holla at arm.com>
Reviewed-by: Lukasz Luba <lukasz.luba at arm.com>
Tested-by: Lukasz Luba <lukasz.luba at arm.com>
Tested-by: Ondrej Jirman <megous at megous.com>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann at arm.com>

Changes in v2:

- Removed cosmetic cleanup in pr_info() of store_cpu_topology()
- Guarded remove_cpu_topology() with CONFIG_GENERIC_ARCH_TOPOLOGY

I opted for an #ifdef in __cpu_disable rather than a stub definition of
remove_cpu_topology() in include/linux/arch_topology.h to keep this
change small.

 arch/arm/kernel/smp.c      |  4 ++++
 arch/arm/kernel/topology.c | 10 +++++-----
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 4b0bab2607e4..46e1be9e57a8 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -240,6 +240,10 @@ int __cpu_disable(void)
 	if (ret)
 		return ret;
+	remove_cpu_topology(cpu);
 	 * Take this CPU offline.  Once we clear this, we can't return,
 	 * and we must not schedule until we're ready to give up the cpu.
diff --git a/arch/arm/kernel/topology.c b/arch/arm/kernel/topology.c
index 5b9faba03afb..8d2e61d9e7a6 100644
--- a/arch/arm/kernel/topology.c
+++ b/arch/arm/kernel/topology.c
@@ -196,9 +196,8 @@ void store_cpu_topology(unsigned int cpuid)
 	struct cpu_topology *cpuid_topo = &cpu_topology[cpuid];
 	unsigned int mpidr;
-	/* If the cpu topology has been already set, just return */
-	if (cpuid_topo->core_id != -1)
-		return;
+	if (cpuid_topo->package_id != -1)
+		goto topology_populated;
 	mpidr = read_cpuid_mpidr();
@@ -231,14 +230,15 @@ void store_cpu_topology(unsigned int cpuid)
 		cpuid_topo->package_id = -1;
-	update_siblings_masks(cpuid);
 	pr_info("CPU%u: thread %d, cpu %d, socket %d, mpidr %x\n",
 		cpuid, cpu_topology[cpuid].thread_id,
 		cpu_topology[cpuid].package_id, mpidr);
+	update_siblings_masks(cpuid);
 static inline int cpu_corepower_flags(void)

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