Documentation/arch/arm64/cpu-hotplug.rst | 11 +++++++---- arch/arm64/kernel/acpi.c | 2 ++ arch/arm64/kernel/smp.c | 12 +++++++++++- 3 files changed, 20 insertions(+), 5 deletions(-)
On arm64, when booting with `maxcpus` greater than the number of present
CPUs (e.g., QEMU -smp cpus=4,maxcpus=8), some CPUs are marked as 'present'
but have not yet been registered via register_cpu(). Consequently,
the per-cpu device objects for these CPUs are not yet initialized.
In cpuhp_smt_enable(), the code iterates over all present CPUs. Calling
_cpu_up() for these unregistered CPUs eventually leads to
sysfs_create_group() being called with a NULL kobject (or a kobject
without a directory), triggering the following warning in
fs/sysfs/group.c:
if (WARN_ON(!kobj || (!update && !kobj->sd)))
return -EINVAL;
When booting with ACPI, arm64 smp_prepare_cpus() currently sets all
enumerated CPUs as "present" regardless of their status in the MADT. This
causes issues with SMT hotplug control. For instance, with QEMU's
"-smp 4,maxcpus=8" configuration, the MADT GICC entries are populated as
follows: the first four CPUs are marked Enabled while the remaining four
are marked Online Capable to support potential hot-plugging.
Fix this by:
1. When booting with ACPI, checking the ACPI_MADT_ENABLED flag in the GICC
entry before calling set_cpu_present() during SMP initialization.
2. Properly managing the present mask in acpi_map_cpu() and
acpi_unmap_cpu() to support actual CPU hotplug events, This aligns with
other architectures like x86 and LoongArch.
3. Update the arm64 CPU hotplug documentation to no longer state that all
online-capable vCPUs are marked as present by the kernel at boot time.
This ensures that only physically available or explicitly enabled CPUs
are in the present mask, keeping the SMT control logic consistent with
the actual hardware state.
How to reproduce:
1. echo off > /sys/devices/system/cpu/smt/control
psci: CPU1 killed (polled 0 ms)
psci: CPU3 killed (polled 0 ms)
2. echo 2 > /sys/devices/system/cpu/smt/control
Detected PIPT I-cache on CPU1
GICv3: CPU1: found redistributor 1 region 0:0x00000000080c0000
CPU1: Booted secondary processor 0x0000000001 [0x410fd082]
Detected PIPT I-cache on CPU3
GICv3: CPU3: found redistributor 3 region 0:0x0000000008100000
CPU3: Booted secondary processor 0x0000000003 [0x410fd082]
------------[ cut here ]------------
WARNING: fs/sysfs/group.c:137 at internal_create_group+0x41c/0x4bc, CPU#2: sh/181
Modules linked in:
CPU: 2 UID: 0 PID: 181 Comm: sh Not tainted 7.0.0-rc1-00010-g8d13386c7624 #142 PREEMPT
Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : internal_create_group+0x41c/0x4bc
lr : sysfs_create_group+0x18/0x24
sp : ffff80008078ba40
x29: ffff80008078ba40 x28: ffff296c980ad000 x27: ffff00007fb94128
x26: 0000000000000054 x25: ffffd693e845f3f0 x24: 0000000000000001
x23: 0000000000000001 x22: 0000000000000004 x21: 0000000000000000
x20: ffffd693e845fc10 x19: 0000000000000004 x18: 00000000ffffffff
x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
x14: 0000000000000358 x13: 0000000000000007 x12: 0000000000000350
x11: 0000000000000008 x10: 0000000000000407 x9 : 0000000000000400
x8 : ffff00007fbf3b60 x7 : 0000000000000000 x6 : ffffd693e845f3f0
x5 : ffff00007fb94128 x4 : 0000000000000000 x3 : ffff000000f4eac0
x2 : ffffd693e7095a08 x1 : 0000000000000000 x0 : 0000000000000000
Call trace:
internal_create_group+0x41c/0x4bc (P)
sysfs_create_group+0x18/0x24
topology_add_dev+0x1c/0x28
cpuhp_invoke_callback+0x104/0x20c
__cpuhp_invoke_callback_range+0x94/0x11c
_cpu_up+0x200/0x37c
cpuhp_smt_enable+0xbc/0x114
control_store+0xe8/0x1d4
dev_attr_store+0x18/0x2c
sysfs_kf_write+0x7c/0x94
kernfs_fop_write_iter+0x128/0x1b8
vfs_write+0x2b0/0x354
ksys_write+0x68/0xfc
__arm64_sys_write+0x1c/0x28
invoke_syscall+0x48/0x10c
el0_svc_common.constprop.0+0x40/0xe8
do_el0_svc+0x20/0x2c
el0_svc+0x34/0x124
el0t_64_sync_handler+0xa0/0xe4
el0t_64_sync+0x198/0x19c
---[ end trace 0000000000000000 ]---
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Cc: stable@vger.kernel.org
Link: https://uefi.org/specs/ACPI/6.5/05_ACPI_Software_Programming_Model.html#gic-cpu-interface-gicc-structure
Fixes: eed4583bcf9a6 ("arm64: Kconfig: Enable HOTPLUG_SMT")
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
---
v3:
- Update the arm64 cpu-hotplug documentation as Catalin suggested.
- Update the commit message.
v2:
- Update the fix way.
---
Documentation/arch/arm64/cpu-hotplug.rst | 11 +++++++----
arch/arm64/kernel/acpi.c | 2 ++
arch/arm64/kernel/smp.c | 12 +++++++++++-
3 files changed, 20 insertions(+), 5 deletions(-)
diff --git a/Documentation/arch/arm64/cpu-hotplug.rst b/Documentation/arch/arm64/cpu-hotplug.rst
index 8fb438bf7781..60f7f51d7b96 100644
--- a/Documentation/arch/arm64/cpu-hotplug.rst
+++ b/Documentation/arch/arm64/cpu-hotplug.rst
@@ -47,8 +47,9 @@ ever have can be described at boot. There are no power-domain considerations
as such devices are emulated.
CPU Hotplug on virtual systems is supported. It is distinct from physical
-CPU Hotplug as all resources are described as ``present``, but CPUs may be
-marked as disabled by firmware. Only the CPU's online/offline behaviour is
+CPU Hotplug as all resources are described in the static configuration tables,
+but vCPUs that are not enabled at boot are not marked as ``present`` by the
+kernel until they are hotplugged. Only the CPU's online/offline behaviour is
influenced by firmware. An example is where a virtual machine boots with a
single CPU, and additional CPUs are added once a cloud orchestrator deploys
the workload.
@@ -68,8 +69,10 @@ redistributors.
CPUs described as ``online capable`` but not ``enabled`` can be set to enabled
by the DSDT's Processor object's _STA method. On virtual systems the _STA method
-must always report the CPU as ``present``. Changes to the firmware policy can
-be notified to the OS via device-check or eject-request.
+must report the CPU as ``present`` when it is activated by the firmware.
+The kernel will then set the vCPU as ``present`` dynamically during the hotplug
+configuration process. Changes can be notified to the OS via device-check or
+eject-request.
CPUs described as ``enabled`` in the static table, should not have their _STA
modified dynamically by firmware. Soft-restart features such as kexec will
diff --git a/arch/arm64/kernel/acpi.c b/arch/arm64/kernel/acpi.c
index 5891f92c2035..681aa2bbc399 100644
--- a/arch/arm64/kernel/acpi.c
+++ b/arch/arm64/kernel/acpi.c
@@ -448,12 +448,14 @@ int acpi_map_cpu(acpi_handle handle, phys_cpuid_t physid, u32 apci_id,
return *pcpu;
}
+ set_cpu_present(*pcpu, true);
return 0;
}
EXPORT_SYMBOL(acpi_map_cpu);
int acpi_unmap_cpu(int cpu)
{
+ set_cpu_present(cpu, false);
return 0;
}
EXPORT_SYMBOL(acpi_unmap_cpu);
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 1aa324104afb..5932e5b30b71 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -566,6 +566,11 @@ struct acpi_madt_generic_interrupt *acpi_cpu_get_madt_gicc(int cpu)
}
EXPORT_SYMBOL_GPL(acpi_cpu_get_madt_gicc);
+static bool acpi_cpu_is_present(int cpu)
+{
+ return acpi_cpu_get_madt_gicc(cpu)->flags & ACPI_MADT_ENABLED;
+}
+
/*
* acpi_map_gic_cpu_interface - parse processor MADT entry
*
@@ -670,6 +675,10 @@ static void __init acpi_parse_and_init_cpus(void)
early_map_cpu_to_node(i, acpi_numa_get_nid(i));
}
#else
+static bool acpi_cpu_is_present(int cpu)
+{
+ return false;
+}
#define acpi_parse_and_init_cpus(...) do { } while (0)
#endif
@@ -808,7 +817,8 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
if (err)
continue;
- set_cpu_present(cpu, true);
+ if (acpi_disabled || acpi_cpu_is_present(cpu))
+ set_cpu_present(cpu, true);
numa_store_cpu_info(cpu);
}
}
--
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