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

Waiman Long
Mon, 27 Jun 2022 13:25:54 -0000

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