Re: [PATCH v6] x86/paravirt: useless assignment instructions cause Unixbench full core performance degradation

[Waiman Long <longman@redhat.com>]

On 6/28/22 08:54, Guo Hui wrote:
> 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 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            | 12 +++++++++++-
>   2 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/paravirt-spinlocks.c b/arch/x86/kernel/paravirt-spinlocks.c
> index 9e1ea99..a2eb375 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_TRUE(vcpu_has_preemption);
> +
>   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_disable(&vcpu_has_preemption);
>   }
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index d5610ad..883e815 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -27,6 +27,16 @@ static inline int node_cpu(struct optimistic_spin_node *node)
>   	return node->cpu - 1;
>   }
>   
> +DEFINE_STATIC_KEY_TRUE(vcpu_has_preemption);
> +
> +static inline bool vcpu_is_preempted_node(struct optimistic_spin_node *node)
> +{
> +	if (static_branch_likely(&vcpu_has_preemption))
> +		return vcpu_is_preempted(node_cpu(node->prev));
> +
> +	return false;
> +}
> +
>   static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
>   {
>   	int cpu_nr = encoded_cpu_val - 1;
> @@ -141,7 +151,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>   	 * polling, be careful.
>   	 */
>   	if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
> -				  vcpu_is_preempted(node_cpu(node->prev))))
> +						vcpu_is_preempted_node(node)))
>   		return true;
>   
>   	/* unqueue */

How about a further improvement on configurations that don't use 
vcpu_is_preempted() at all?

+#ifdef vcpu_is_preempted
+DEFINE_STATIC_KEY_TRUE(vcpu_has_preemption);
+
  static inline int node_cpu(struct optimistic_spin_node *node)
  {
         return node->cpu - 1;
  }

+static inline bool vcpu_is_preempted_node(struct optimistic_spin_node 
*node)
+{
+       if (static_branch_likely(&vcpu_has_preemption))
+               return vcpu_is_preempted(node_cpu(node->prev));
+
+       return false;
+}
+#else
+static inline bool vcpu_is_preempted_node(struct optimistic_spin_node 
*node)
+{
+       return false;
+}
+#endif
+
  static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
  {
         int cpu_nr = encoded_cpu_val - 1;
@@ -141,7 +158,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
          * polling, be careful.
          */
         if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
- vcpu_is_preempted(node_cpu(node->prev))))
+                                 vcpu_is_preempted_node(node)))
                 return true;

         /* unqueue */

For those configurations, vcpu_is_preempted_node() will just get 
compiled out.

Cheers,
Longman