[PATCH v2] x86/paravirt: useless assignment instructions cause Unixbench full core performance degradation

[Guo Hui <guohui@uniontech.com>]

The instructions assigned to the vcpu_is_preempted function parameter
in the X86 architecture physical machine are redundant instructions,
causing the multi-core performance of Unixbench to drop by about 4% to 5%.
The C function is as follows:
static bool vcpu_is_preempted(long vcpu);

The parameter 'vcpu' in the function osq_lock
that calls the function vcpu_is_preempted is assigned as follows:

The C code is in the function node_cpu:
cpu = node->cpu - 1;

The instructions corresponding to the C code are:
mov 0x14(%rax),%edi
sub $0x1,%edi

The above instructions are unnecessary
in the X86 Native operating environment,
causing high cache-misses and degrading performance.

This patch uses static_key to not execute this instruction
in the Native runtime environment.

The code is as follows:

DEFINE_STATIC_KEY_FALSE(preemted_key);

static inline int node_cpu(struct optimistic_spin_node *node)
{
     int cpu = 0;

     if (!static_branch_unlikely(&preemted_key))
             cpu = node->cpu - 1;

     return cpu;
}

The patch effect is as follows two machines,
Unixbench runs with full core score:

1. Machine configuration:
Intel(R) Xeon(R) Silver 4210 CPU @ 2.20GHz
CPU core: 40
Memory: 256G
OS Kernel: 5.19-rc3

Before using the patch:
System Benchmarks Index Values               BASELINE       RESULT    INDEX
Dhrystone 2 using register variables         116700.0  948326591.2  81261.9
Double-Precision Whetstone                       55.0     211986.3  38543.0
Execl Throughput                                 43.0      43453.2  10105.4
File Copy 1024 bufsize 2000 maxblocks          3960.0     438936.2   1108.4
File Copy 256 bufsize 500 maxblocks            1655.0     118197.4    714.2
File Copy 4096 bufsize 8000 maxblocks          5800.0    1534674.7   2646.0
Pipe Throughput                               12440.0   46482107.6  37365.0
Pipe-based Context Switching                   4000.0    1915094.2   4787.7
Process Creation                                126.0      85442.2   6781.1
Shell Scripts (1 concurrent)                     42.4      69400.7  16368.1
Shell Scripts (8 concurrent)                      6.0       8877.2  14795.3
System Call Overhead                          15000.0    4714906.1   3143.3
                                                                   ========
System Benchmarks Index Score                                        7923.3

After using the patch:
System Benchmarks Index Values               BASELINE       RESULT    INDEX
Dhrystone 2 using register variables         116700.0  947032915.5  81151.1
Double-Precision Whetstone                       55.0     211971.2  38540.2
Execl Throughput                                 43.0      45054.8  10477.9
File Copy 1024 bufsize 2000 maxblocks          3960.0     515024.9   1300.6
File Copy 256 bufsize 500 maxblocks            1655.0     146354.6    884.3
File Copy 4096 bufsize 8000 maxblocks          5800.0    1679995.9   2896.5
Pipe Throughput                               12440.0   46466394.2  37352.4
Pipe-based Context Switching                   4000.0    1898221.4   4745.6
Process Creation                                126.0      85653.1   6797.9
Shell Scripts (1 concurrent)                     42.4      69437.3  16376.7
Shell Scripts (8 concurrent)                      6.0       8898.9  14831.4
System Call Overhead                          15000.0    4658746.7   3105.8
                                                                   ========
System Benchmarks Index Score                                        8248.8

2. Machine configuration:
Hygon C86 7185 32-core Processor
CPU core: 128
Memory: 256G
OS Kernel: 5.19-rc3

Before using the patch:
System Benchmarks Index Values               BASELINE       RESULT    INDEX
Dhrystone 2 using register variables         116700.0 2256644068.3 193371.4
Double-Precision Whetstone                       55.0     438969.9  79812.7
Execl Throughput                                 43.0      10108.6   2350.8
File Copy 1024 bufsize 2000 maxblocks          3960.0     275892.8    696.7
File Copy 256 bufsize 500 maxblocks            1655.0      72082.7    435.5
File Copy 4096 bufsize 8000 maxblocks          5800.0     925043.4   1594.9
Pipe Throughput                               12440.0  118905512.5  95583.2
Pipe-based Context Switching                   4000.0    7820945.7  19552.4
Process Creation                                126.0      31233.3   2478.8
Shell Scripts (1 concurrent)                     42.4      49042.8  11566.7
Shell Scripts (8 concurrent)                      6.0       6656.0  11093.3
System Call Overhead                          15000.0    6816047.5   4544.0
                                                                   ========
System Benchmarks Index Score                                        7756.6

After using the patch:
System Benchmarks Index Values               BASELINE       RESULT    INDEX
Dhrystone 2 using register variables         116700.0 2252272929.4 192996.8
Double-Precision Whetstone                       55.0     451847.2  82154.0
Execl Throughput                                 43.0      10595.1   2464.0
File Copy 1024 bufsize 2000 maxblocks          3960.0     301279.3    760.8
File Copy 256 bufsize 500 maxblocks            1655.0      79291.3    479.1
File Copy 4096 bufsize 8000 maxblocks          5800.0    1039755.2   1792.7
Pipe Throughput                               12440.0  118701468.1  95419.2
Pipe-based Context Switching                   4000.0    8073453.3  20183.6
Process Creation                                126.0      33440.9   2654.0
Shell Scripts (1 concurrent)                     42.4      52722.6  12434.6
Shell Scripts (8 concurrent)                      6.0       7050.4  11750.6
System Call Overhead                          15000.0    6834371.5   4556.2
                                                                   ========
System Benchmarks Index Score                                        8157.8

Signed-off-by: Guo Hui <guohui@uniontech.com>
---
 arch/x86/kernel/paravirt-spinlocks.c | 4 ++++
 kernel/locking/osq_lock.c            | 9 ++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/paravirt-spinlocks.c b/arch/x86/kernel/paravirt-spinlocks.c
index 9e1ea99ad..7a55f8407 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -33,6 +33,8 @@ bool pv_is_native_vcpu_is_preempted(void)
 		__raw_callee_save___native_vcpu_is_preempted;
 }
 
+DECLARE_STATIC_KEY_FALSE(preemted_key);
+
 void __init paravirt_set_cap(void)
 {
 	if (!pv_is_native_spin_unlock())
@@ -40,4 +42,6 @@ void __init paravirt_set_cap(void)
 
 	if (!pv_is_native_vcpu_is_preempted())
 		setup_force_cpu_cap(X86_FEATURE_VCPUPREEMPT);
+	else
+		static_branch_enable(&preemted_key);
 }
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index d5610ad52..a8798e701 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -22,9 +22,16 @@ static inline int encode_cpu(int cpu_nr)
 	return cpu_nr + 1;
 }
 
+DEFINE_STATIC_KEY_FALSE(preemted_key);
+
 static inline int node_cpu(struct optimistic_spin_node *node)
 {
-	return node->cpu - 1;
+	int cpu = 0;
+
+	if (!static_branch_unlikely(&preemted_key))
+		cpu = node->cpu - 1;
+
+	return cpu;
 }
 
 static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
-- 
2.20.1