[PATCH] LoongArch: Revert qspinlock to test-and-set on VMs

[Bibo Mao]

Similar with x86, when VM is detected, revert to a simple test-and-set
lock to avoid the horrors of queue preemption.

Tested on 3C5000 Dual-way machine with 32 cores and 2 numa nodes,
test case is kcbench on kernel mainline 6.10, the detailed command is
"kcbench --src /root/src/linux"

Performance on host machine
                      kernel compile time       performance impact
   Original           150.29 seconds
   With patch         150.19 seconds            almost no impact

Performance on virtual machine:
1. 1 VM with 32 vCPUs and 2 numa node, numa node pinned
                      kernel compile time       performance impact
   Original           170.87 seconds
   With patch         171.73 seconds            almost no impact

2. 2 VMs, each VM with 32 vCPUs and 2 numa node, numa node pinned
                      kernel compile time       performance impact
   Original           2362.04 seconds
   With patch         354.73  seconds            +565%

Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
---
 arch/loongarch/include/asm/Kbuild      |  1 -
 arch/loongarch/include/asm/paravirt.h  |  3 ++
 arch/loongarch/include/asm/qspinlock.h | 41 ++++++++++++++++++++++++++
 arch/loongarch/kernel/paravirt.c       |  9 ++++++
 arch/loongarch/kernel/setup.c          |  5 ++++
 arch/loongarch/kernel/smp.c            |  2 ++
 6 files changed, 60 insertions(+), 1 deletion(-)
 create mode 100644 arch/loongarch/include/asm/qspinlock.h

diff --git a/arch/loongarch/include/asm/Kbuild b/arch/loongarch/include/asm/Kbuild
index 2bb3676429c0..4635b755b2b4 100644
--- a/arch/loongarch/include/asm/Kbuild
+++ b/arch/loongarch/include/asm/Kbuild
@@ -6,7 +6,6 @@ generic-y += mcs_spinlock.h
 generic-y += parport.h
 generic-y += early_ioremap.h
 generic-y += qrwlock.h
-generic-y += qspinlock.h
 generic-y += user.h
 generic-y += ioctl.h
 generic-y += statfs.h
diff --git a/arch/loongarch/include/asm/paravirt.h b/arch/loongarch/include/asm/paravirt.h
index dddec49671ae..dcc2b46d31fe 100644
--- a/arch/loongarch/include/asm/paravirt.h
+++ b/arch/loongarch/include/asm/paravirt.h
@@ -19,6 +19,7 @@ static inline u64 paravirt_steal_clock(int cpu)
 
 int __init pv_ipi_init(void);
 int __init pv_time_init(void);
+void __init pv_spinlock_init(void);
 
 #else
 
@@ -31,5 +32,7 @@ static inline int pv_time_init(void)
 {
 	return 0;
 }
+
+static inline void pv_spinlock_init(void) { }
 #endif // CONFIG_PARAVIRT
 #endif
diff --git a/arch/loongarch/include/asm/qspinlock.h b/arch/loongarch/include/asm/qspinlock.h
new file mode 100644
index 000000000000..b2d53b8c6679
--- /dev/null
+++ b/arch/loongarch/include/asm/qspinlock.h
@@ -0,0 +1,41 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_LOONGARCH_QSPINLOCK_H
+#define _ASM_LOONGARCH_QSPINLOCK_H
+
+#include <linux/jump_label.h>
+#include <asm/cpu-features.h>
+
+#ifdef CONFIG_PARAVIRT
+
+DECLARE_STATIC_KEY_TRUE(virt_spin_lock_key);
+
+#define virt_spin_lock virt_spin_lock
+static inline bool virt_spin_lock(struct qspinlock *lock)
+{
+	int val;
+
+	if (static_branch_likely(&virt_spin_lock_key))
+		return false;
+
+	/*
+	 * On hypervisors without PARAVIRT_SPINLOCKS support we fall
+	 * back to a Test-and-Set spinlock, because fair locks have
+	 * horrible lock 'holder' preemption issues.
+	 */
+
+__retry:
+	val = atomic_read(&lock->val);
+
+	if (val || !atomic_try_cmpxchg(&lock->val, &val, _Q_LOCKED_VAL)) {
+		cpu_relax();
+		goto __retry;
+	}
+
+	return true;
+}
+
+#endif /* CONFIG_PARAVIRT */
+
+#include <asm-generic/qspinlock.h>
+
+#endif // _ASM_LOONGARCH_QSPINLOCK_H
diff --git a/arch/loongarch/kernel/paravirt.c b/arch/loongarch/kernel/paravirt.c
index 9c9b75b76f62..49ebc54bdbcb 100644
--- a/arch/loongarch/kernel/paravirt.c
+++ b/arch/loongarch/kernel/paravirt.c
@@ -9,6 +9,7 @@
 #include <linux/static_call.h>
 #include <asm/paravirt.h>
 
+DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
 static int has_steal_clock;
 struct static_key paravirt_steal_enabled;
 struct static_key paravirt_steal_rq_enabled;
@@ -300,3 +301,11 @@ int __init pv_time_init(void)
 
 	return 0;
 }
+
+void __init pv_spinlock_init(void)
+{
+	if (!cpu_has_hypervisor)
+		return;
+
+	static_branch_disable(&virt_spin_lock_key);
+}
diff --git a/arch/loongarch/kernel/setup.c b/arch/loongarch/kernel/setup.c
index 0f0740f0be27..70a670efe3cf 100644
--- a/arch/loongarch/kernel/setup.c
+++ b/arch/loongarch/kernel/setup.c
@@ -599,6 +599,11 @@ void __init setup_arch(char **cmdline_p)
 	parse_early_param();
 	reserve_initrd_mem();
 
+	/*
+	 * Initialise the static keys early as they may be enabled by the
+	 * cpufeature code and early parameters.
+	 */
+	jump_label_init();
 	platform_init();
 	arch_mem_init(cmdline_p);
 
diff --git a/arch/loongarch/kernel/smp.c b/arch/loongarch/kernel/smp.c
index ca405ab86aae..f499bff1050b 100644
--- a/arch/loongarch/kernel/smp.c
+++ b/arch/loongarch/kernel/smp.c
@@ -509,6 +509,8 @@ void smp_prepare_boot_cpu(void)
 			rr_node = next_node_in(rr_node, node_online_map);
 		}
 	}
+
+	pv_spinlock_init();
 }
 
 /* called from main before smp_init() */

base-commit: 94ede2a3e9135764736221c080ac7c0ad993dc2d
-- 
2.39.3