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Subject[PATCH v11 11/11] RISC-V: Support cpu hotplug
Date
This patch enable support for cpu hotplug in RISC-V. It uses SBI HSM
extension to online/offline any hart. As a result, the harts are
returned to firmware once they are offline. If the harts are brought
online afterwards, they re-enter Linux kernel as if a secondary hart
booted for the first time. All booting requirements are honored during
this process.

Tested both on QEMU and HighFive Unleashed board with. Test result follows.

---------------------------------------------------
Offline cpu 2
---------------------------------------------------
$ echo 0 > /sys/devices/system/cpu/cpu2/online
[ 32.828684] CPU2: off
$ cat /proc/cpuinfo
processor : 0
hart : 0
isa : rv64imafdcsu
mmu : sv48

processor : 1
hart : 1
isa : rv64imafdcsu
mmu : sv48

processor : 3
hart : 3
isa : rv64imafdcsu
mmu : sv48

processor : 4
hart : 4
isa : rv64imafdcsu
mmu : sv48

processor : 5
hart : 5
isa : rv64imafdcsu
mmu : sv48

processor : 6
hart : 6
isa : rv64imafdcsu
mmu : sv48

processor : 7
hart : 7
isa : rv64imafdcsu
mmu : sv48

---------------------------------------------------
online cpu 2
---------------------------------------------------
$ echo 1 > /sys/devices/system/cpu/cpu2/online
$ cat /proc/cpuinfo
processor : 0
hart : 0
isa : rv64imafdcsu
mmu : sv48

processor : 1
hart : 1
isa : rv64imafdcsu
mmu : sv48

processor : 2
hart : 2
isa : rv64imafdcsu
mmu : sv48

processor : 3
hart : 3
isa : rv64imafdcsu
mmu : sv48

processor : 4
hart : 4
isa : rv64imafdcsu
mmu : sv48

processor : 5
hart : 5
isa : rv64imafdcsu
mmu : sv48

processor : 6
hart : 6
isa : rv64imafdcsu
mmu : sv48

processor : 7
hart : 7
isa : rv64imafdcsu
mmu : sv48

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
---
arch/riscv/Kconfig | 12 ++++-
arch/riscv/include/asm/cpu_ops.h | 12 +++++
arch/riscv/include/asm/smp.h | 17 +++++++
arch/riscv/kernel/Makefile | 1 +
arch/riscv/kernel/cpu-hotplug.c | 87 ++++++++++++++++++++++++++++++++
arch/riscv/kernel/cpu_ops_sbi.c | 34 +++++++++++++
arch/riscv/kernel/setup.c | 19 ++++++-
7 files changed, 180 insertions(+), 2 deletions(-)
create mode 100644 arch/riscv/kernel/cpu-hotplug.c

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 20c6191399ea..b3daadd116ec 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -20,7 +20,6 @@ config RISCV
select CLONE_BACKWARDS
select COMMON_CLK
select GENERIC_CLOCKEVENTS
- select GENERIC_CPU_DEVICES
select GENERIC_IRQ_SHOW
select GENERIC_PCI_IOMAP
select GENERIC_SCHED_CLOCK
@@ -248,6 +247,17 @@ config NR_CPUS
depends on SMP
default "8"

+config HOTPLUG_CPU
+ bool "Support for hot-pluggable CPUs"
+ depends on SMP
+ select GENERIC_IRQ_MIGRATION
+ help
+
+ Say Y here to experiment with turning CPUs off and on. CPUs
+ can be controlled through /sys/devices/system/cpu.
+
+ Say N if you want to disable CPU hotplug.
+
choice
prompt "CPU Tuning"
default TUNE_GENERIC
diff --git a/arch/riscv/include/asm/cpu_ops.h b/arch/riscv/include/asm/cpu_ops.h
index 5ce81a28e1d9..a8ec3c5c1bd2 100644
--- a/arch/riscv/include/asm/cpu_ops.h
+++ b/arch/riscv/include/asm/cpu_ops.h
@@ -18,12 +18,24 @@
* is a mechanism for doing so, tests whether it is
* possible to boot the given HART.
* @cpu_start: Boots a cpu into the kernel.
+ * @cpu_disable: Prepares a cpu to die. May fail for some
+ * mechanism-specific reason, which will cause the hot
+ * unplug to be aborted. Called from the cpu to be killed.
+ * @cpu_stop: Makes a cpu leave the kernel. Must not fail. Called from
+ * the cpu being stopped.
+ * @cpu_is_stopped: Ensures a cpu has left the kernel. Called from another
+ * cpu.
*/
struct cpu_operations {
const char *name;
int (*cpu_prepare)(unsigned int cpu);
int (*cpu_start)(unsigned int cpu,
struct task_struct *tidle);
+#ifdef CONFIG_HOTPLUG_CPU
+ int (*cpu_disable)(unsigned int cpu);
+ void (*cpu_stop)(void);
+ int (*cpu_is_stopped)(unsigned int cpu);
+#endif
};

extern const struct cpu_operations *cpu_ops[NR_CPUS];
diff --git a/arch/riscv/include/asm/smp.h b/arch/riscv/include/asm/smp.h
index 023f74fb8b3b..f4c7cfda6b7f 100644
--- a/arch/riscv/include/asm/smp.h
+++ b/arch/riscv/include/asm/smp.h
@@ -43,6 +43,13 @@ void riscv_cpuid_to_hartid_mask(const struct cpumask *in, struct cpumask *out);
*/
#define raw_smp_processor_id() (current_thread_info()->cpu)

+#if defined CONFIG_HOTPLUG_CPU
+int __cpu_disable(void);
+void __cpu_die(unsigned int cpu);
+void cpu_stop(void);
+#else
+#endif /* CONFIG_HOTPLUG_CPU */
+
#else

static inline void show_ipi_stats(struct seq_file *p, int prec)
@@ -69,4 +76,14 @@ static inline void riscv_cpuid_to_hartid_mask(const struct cpumask *in,
}

#endif /* CONFIG_SMP */
+
+#if defined(CONFIG_HOTPLUG_CPU) && (CONFIG_SMP)
+bool cpu_has_hotplug(unsigned int cpu);
+#else
+static inline bool cpu_has_hotplug(unsigned int cpu)
+{
+ return false;
+}
+#endif
+
#endif /* _ASM_RISCV_SMP_H */
diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index a0be34b96846..9601ac907f70 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -47,5 +47,6 @@ obj-$(CONFIG_RISCV_SBI) += sbi.o
ifeq ($(CONFIG_RISCV_SBI), y)
obj-$(CONFIG_SMP) += cpu_ops_sbi.o
endif
+obj-$(CONFIG_HOTPLUG_CPU) += cpu-hotplug.o

clean:
diff --git a/arch/riscv/kernel/cpu-hotplug.c b/arch/riscv/kernel/cpu-hotplug.c
new file mode 100644
index 000000000000..df84e0c13db1
--- /dev/null
+++ b/arch/riscv/kernel/cpu-hotplug.c
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2020 Western Digital Corporation or its affiliates.
+ */
+
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/sched.h>
+#include <linux/err.h>
+#include <linux/irq.h>
+#include <linux/cpu.h>
+#include <linux/sched/hotplug.h>
+#include <asm/irq.h>
+#include <asm/cpu_ops.h>
+#include <asm/sbi.h>
+
+void cpu_stop(void);
+void arch_cpu_idle_dead(void)
+{
+ cpu_stop();
+}
+
+bool cpu_has_hotplug(unsigned int cpu)
+{
+ if (cpu_ops[cpu]->cpu_stop)
+ return true;
+
+ return false;
+}
+
+/*
+ * __cpu_disable runs on the processor to be shutdown.
+ */
+int __cpu_disable(void)
+{
+ int ret = 0;
+ unsigned int cpu = smp_processor_id();
+
+ if (!cpu_ops[cpu] || !cpu_ops[cpu]->cpu_stop)
+ return -EOPNOTSUPP;
+
+ if (cpu_ops[cpu]->cpu_disable)
+ ret = cpu_ops[cpu]->cpu_disable(cpu);
+
+ if (ret)
+ return ret;
+
+ remove_cpu_topology(cpu);
+ set_cpu_online(cpu, false);
+ irq_migrate_all_off_this_cpu();
+
+ return ret;
+}
+
+/*
+ * Called on the thread which is asking for a CPU to be shutdown.
+ */
+void __cpu_die(unsigned int cpu)
+{
+ int ret = 0;
+
+ if (!cpu_wait_death(cpu, 5)) {
+ pr_err("CPU %u: didn't die\n", cpu);
+ return;
+ }
+ pr_notice("CPU%u: off\n", cpu);
+
+ /* Verify from the firmware if the cpu is really stopped*/
+ if (cpu_ops[cpu]->cpu_is_stopped)
+ ret = cpu_ops[cpu]->cpu_is_stopped(cpu);
+ if (ret)
+ pr_warn("CPU%d may not have stopped: %d\n", cpu, ret);
+}
+
+/*
+ * Called from the idle thread for the CPU which has been shutdown.
+ */
+void cpu_stop(void)
+{
+ idle_task_exit();
+
+ (void)cpu_report_death();
+
+ cpu_ops[smp_processor_id()]->cpu_stop();
+ /* It should never reach here */
+ BUG();
+}
diff --git a/arch/riscv/kernel/cpu_ops_sbi.c b/arch/riscv/kernel/cpu_ops_sbi.c
index 66f3cded91f5..685fae72b7f5 100644
--- a/arch/riscv/kernel/cpu_ops_sbi.c
+++ b/arch/riscv/kernel/cpu_ops_sbi.c
@@ -74,8 +74,42 @@ static int sbi_cpu_prepare(unsigned int cpuid)
return 0;
}

+#ifdef CONFIG_HOTPLUG_CPU
+static int sbi_cpu_disable(unsigned int cpuid)
+{
+ if (!cpu_ops_sbi.cpu_stop)
+ return -EOPNOTSUPP;
+ return 0;
+}
+
+static void sbi_cpu_stop(void)
+{
+ int ret;
+
+ ret = sbi_hsm_hart_stop();
+ pr_crit("Unable to stop the cpu %u (%d)\n", smp_processor_id(), ret);
+}
+
+static int sbi_cpu_is_stopped(unsigned int cpuid)
+{
+ int rc;
+ int hartid = cpuid_to_hartid_map(cpuid);
+
+ rc = sbi_hsm_hart_get_status(hartid);
+
+ if (rc == SBI_HSM_HART_STATUS_STOPPED)
+ return 0;
+ return rc;
+}
+#endif
+
const struct cpu_operations cpu_ops_sbi = {
.name = "sbi",
.cpu_prepare = sbi_cpu_prepare,
.cpu_start = sbi_cpu_start,
+#ifdef CONFIG_HOTPLUG_CPU
+ .cpu_disable = sbi_cpu_disable,
+ .cpu_stop = sbi_cpu_stop,
+ .cpu_is_stopped = sbi_cpu_is_stopped,
+#endif
};
diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
index 44ac4ddf60ab..1d2f66579b94 100644
--- a/arch/riscv/kernel/setup.c
+++ b/arch/riscv/kernel/setup.c
@@ -16,12 +16,13 @@
#include <linux/of_platform.h>
#include <linux/sched/task.h>
#include <linux/swiotlb.h>
+#include <linux/smp.h>

#include <asm/clint.h>
+#include <asm/cpu_ops.h>
#include <asm/setup.h>
#include <asm/sections.h>
#include <asm/pgtable.h>
-#include <asm/smp.h>
#include <asm/sbi.h>
#include <asm/tlbflush.h>
#include <asm/thread_info.h>
@@ -43,6 +44,7 @@ struct screen_info screen_info = {
/* The lucky hart to first increment this variable will boot the other cores */
atomic_t hart_lottery;
unsigned long boot_cpu_hartid;
+static DEFINE_PER_CPU(struct cpu, cpu_devices);

void __init parse_dtb(void)
{
@@ -90,3 +92,18 @@ void __init setup_arch(char **cmdline_p)

riscv_fill_hwcap();
}
+
+static int __init topology_init(void)
+{
+ int i;
+
+ for_each_possible_cpu(i) {
+ struct cpu *cpu = &per_cpu(cpu_devices, i);
+
+ cpu->hotpluggable = cpu_has_hotplug(i);
+ register_cpu(cpu, i);
+ }
+
+ return 0;
+}
+subsys_initcall(topology_init);
--
2.25.1
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