On 2024/12/24 0:34, Jonathan Cameron wrote:

On Fri, 20 Dec 2024 15:59:27 +0800
Yicong Yang [off-list ref] wrote:

quoted

On 2024/12/20 15:53, Yicong Yang wrote:

quoted

From: Yicong Yang <yangyicong@hisilicon.com>

Currently if architectures want to support HOTPLUG_SMT they need to
provide a topology_is_primary_thread() telling the framework which
thread in the SMT cannot offline. However arm64 doesn't have a
restriction on which thread in the SMT cannot offline, a simplest
choice is that just make 1st thread as the "primary" thread. So
just make this as the default implementation in the framework and
let architectures like x86 that have special primary thread to
override this function (which they've already done).

There's no need to provide a stub function if !CONFIG_SMP or
!CONFIG_HOTPLUG_SMP. In such case the testing CPU is already
the 1st CPU in the SMT so it's always the primary thread.

Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
---
As questioned in v9 [1] whether this works on architectures not using
CONFIG_GENERIC_ARCH_TOPOLOGY, hacked on LoongArch VM and this also works.
Architectures should use this on their own situation.
[1] https://lore.kernel.org/linux-arm-kernel/427bd639-33c3-47e4-9e83-68c428eb1a7d@arm.com/ (local)

[root@localhost smt]# uname -m
loongarch64
[root@localhost smt]# pwd
/sys/devices/system/cpu/smt
[root@localhost smt]# cat ../possible 
0-3
[root@localhost smt]# cat ../online 
0-3
[root@localhost smt]# cat control 
on
[root@localhost smt]# echo off > control 
[root@localhost smt]# cat control 
off
[root@localhost smt]# cat ../online 
0,2
[root@localhost smt]# echo on > control 
[root@localhost smt]# cat control 
on
[root@localhost smt]# cat ../online 
0-3  

Tested with below code using the topology_is_primary_thread() introduced
in this patch. Tested on an ACPI-based QEMU VM emulating SMT2.

Nice bit of testing.

Given it all seems fine. FWIW
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
(for original patch, not the longarch one!)

thanks. certainly :)

quoted

Subject: [PATCH] LoongArch: Support HOTPLUG_SMT on ACPI-based system

Support HOTPLUG_SMT on ACPI-based system using generic
topology_is_primary_thread().

Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
---
 arch/loongarch/Kconfig       |  1 +
 arch/loongarch/kernel/acpi.c | 26 ++++++++++++++++++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index dae3a9104ca6..bed1b0640b97 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig

@@ -172,6 +172,7 @@ config LOONGARCH
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_TIF_NOHZ
 	select HAVE_VIRT_CPU_ACCOUNTING_GEN if !SMP
+	select HOTPLUG_SMT if HOTPLUG_CPU
 	select IRQ_FORCED_THREADING
 	select IRQ_LOONGARCH_CPU
 	select LOCK_MM_AND_FIND_VMA

diff --git a/arch/loongarch/kernel/acpi.c b/arch/loongarch/kernel/acpi.c
index 382a09a7152c..e642b0de57e7 100644
--- a/arch/loongarch/kernel/acpi.c
+++ b/arch/loongarch/kernel/acpi.c

@@ -15,9 +15,11 @@
 #include <linux/memblock.h>
 #include <linux/of_fdt.h>
 #include <linux/serial_core.h>
+#include <linux/xarray.h>
 #include <asm/io.h>
 #include <asm/numa.h>
 #include <asm/loongson.h>
+#include <linux/cpu_smt.h>

 int acpi_disabled;
 EXPORT_SYMBOL(acpi_disabled);

@@ -175,8 +177,12 @@ int pptt_enabled;

 int __init parse_acpi_topology(void)
 {
+	int thread_num, max_smt_thread_num = 1;
+	struct xarray core_threads;
 	int cpu, topology_id;
+	void *entry;

+	xa_init(&core_threads);
 	for_each_possible_cpu(cpu) {
 		topology_id = find_acpi_cpu_topology(cpu, 0);
 		if (topology_id < 0) {

@@ -184,19 +190,35 @@ int __init parse_acpi_topology(void)
 			return -ENOENT;
 		}

-		if (acpi_pptt_cpu_is_thread(cpu) <= 0)
+		if (acpi_pptt_cpu_is_thread(cpu) <= 0) {
 			cpu_data[cpu].core = topology_id;
-		else {
+		} else {
 			topology_id = find_acpi_cpu_topology(cpu, 1);
 			if (topology_id < 0)
 				return -ENOENT;

 			cpu_data[cpu].core = topology_id;
+
+			entry = xa_load(&core_threads, topology_id);
+			if (!entry) {
+				xa_store(&core_threads, topology_id,
+					 xa_mk_value(1), GFP_KERNEL);
+			} else {
+				thread_num = xa_to_value(entry);
+				thread_num++;
+				xa_store(&core_threads, topology_id,
+					 xa_mk_value(thread_num), GFP_KERNEL);
+
+				if (thread_num > max_smt_thread_num)
+					max_smt_thread_num = thread_num;
+			}
 		}
 	}

 	pptt_enabled = 1;

+	cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
+	xa_destroy(&core_threads);
 	return 0;
 }

.