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

[Waiman Long <longman@redhat.com>]

On 6/27/22 01:54, Guo Hui wrote:
> Thank you very much Longman, my patch is as you said, only disable 
> node_cpu on X86, enable node_cpu on arm64, powerpc, s390 architectures;
> the code is in file arch/x86/kernel/paravirt-spinlocks.c:
>     DECLARE_STATIC_KEY_FALSE(preemted_key);
>     static_branch_enable(&preemted_key);
>
> the default value of preemted_key is false and the if conditional 
> statement is reversed,
> the code is in file kernel/locking/osq_lock.c:
>     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;
>   }
>
> In this way, only one nop instruction is added to architectures arm64, 
> powerpc and s390, including virtual machines, without any other changes.

You are right. I am probably too tired last night to read the patch more 
carefully.

Cheers,
Longman